Surface Science Reports 64 (2009) 233–254

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Surface Science Reports

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Surface modification in microsystems and nanosystems Shaurya Prakash a,b,∗,1, M.B. Karacor a, S. Banerjee a a Department of Mechanical & Aerospace Engineering, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA b Institute for Advanced Materials, Devices, and Nanotechnology, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA article info a b s t r a c t

Article history: Phenomena in microsystems and nanosystems are influenced by the device walls due to the high surface- Accepted 5 May 2009 area-to-volume ratios that are a characteristic feature of these systems. The role of surfaces in these editor: W.H. Weinberg small-scale systems has led to natural interest in developing methods to manipulate surface-mediated phenomena toward improving device performance, developing next generation systems, and mitigat- Keywords: ing problems that arise due to interfacial interactions between surfaces and materials within microscale Surface modification Chemical and nanoscale systems. This report presents a critical review of the existing literature as it relates to role Physical of surfaces and surface modification in microsystems and nanosystems. In addition, this report strives Polymer to present this literature review with an eye on the tutorial aspect of surface modification for new re- Glass searchers. Toward the dual goal of presenting a tutorial review with a critical analysis of literature many Plasma open scientific questions are discussed. Both chemical and physical surface modification methods are dis- Self-assembled monolayer cussed with several examples, applications, and a brief description of underlying theory. The importance Fluid of surfaces in microsystems and nanosystems and the applicability of controlling surface properties in a Micro- systematic manner for both fundamental science and applied studies is also discussed. The readers are Nano- Water purification pointed to several pioneering research efforts over the years that have made surface modification and Energy generation surface science a rich, diverse, and multi-disciplinary research field. It is hoped that this report will as- sist researchers from diverse fields by providing a collection of varied references and encourage the next generation of surface scientists and engineers to significantly advance the state of knowledge. © 2009 Elsevier B.V. All rights reserved. Contents

1. Introduction...... 234 2. Surface modification methods ...... 235 2.1. Overview ...... 235 2.2. Physical methods ...... 235 2.3. Chemical methods ...... 235 3. Thermodynamics of surfaces...... 236 3.1. Overview ...... 236 3.2. Surface layers ...... 237 4. Surface characterization ...... 240 4.1. Indirect methods...... 240 4.2. Direct methods...... 241 5. Applications...... 243 5.1. Photocatalysts for organic decontamination ...... 243 5.2. Dye modified surfaces for energy generation ...... 244 5.3. Microfluidic and nanofluidic separation systems...... 244 5.4. Biosensor platforms...... 247 5.5. Water purification ...... 248 6. Summary ...... 248 Acknowledgements...... 249 References...... 249

∗ Corresponding address: Department of Mechanical & Aerospace Engineering, The State University of New Jersey, Rutgers, B240 Engineering Building, 98 Brett Road, Piscataway, NJ 08854, United States. Tel.: +1 732 445 3797; fax: +1 732 445 1 Current address: Department of Mechanical Engineering, 201 W. 19th Avenue, 3124. Columbus, OH 43210, USA. E-mail address: [email protected] (S. Prakash).

0167-5729/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfrep.2009.05.001 234 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254

Nomenclature θ Fraction of surface sites occupied c Concentration of adsorbing species near the surface in solution or gas-phase b Constant related to adsorbent t Adsorption time ka Adsorption rate constant kd Desorption rate constant N0 Surface adsorbate concentration at full coverage γSV Interfacial free energy between solid and vapor phases γLV Interfacial free energy between liquid and vapor phases γSL Interfacial free energy between solid and liquid Fig. 1. Comparison of papers published in the broad subject area of surface phases modification in microsystems and nanosystems as compared through two commonly used scientific databases. The databases used are Elsevier’s Scopus and θc Contact angle ISI Thomson’s Web of Science. σs Surface charge density ρ Counter-ion charge density in the bulk solution s In light of the definition presented above, surfaces provide a ψ Surface potential s unique set of challenges and opportunities in microsystems and ε Dielectric function of medium nanosystems as these systems are inherently characterized by high ε Permittivity of free-space 0 surface-area-to-volume (SA/V ) ratios varying from about 103 m−1 k Boltzmann constant b to as high as 109 m−1 in some cases. Given these SA/V ratios T Absolute temperature the influence of device and system walls in affecting phenomena ρ Charge density in solution-phase near a surface 0 within confined spaces can no longer be ignored since the walls ζ Zeta potential of surface the of the devices and systems interact directly with the species e Elementary charge in the surface-region. These wall-species interactions that impart λ Debye length or electric double layer thickness D several important and useful characteristics, as discussed later, µ Electroosmotic mobility eo to transport and reaction phenomena in confined microscale and v Electroosmotic velocity eo nanoscale systems. The well-documented [7–13] advantages of φ Applied electric potential microsystems and nanosystems have spawned great interest in η Fluid viscosity developing new materials and devices [14] with wide-ranging R Achievable separation resolution applications including catalysis [15], water purification [16], N Efficiency of separation energy [17], chemical and biological separations [18], personalized ∆µ Difference in ionic mobilities medicine [19], and sensing [20,21]. µ Ion electrophoretic mobility e A system in this report is defined to be a functional unit i.e. it can ∆P Pressure drop be a nanoparticle or an entire lab-on-chip type device. In addition, l Channel or pipe length the ability to control surface properties by attachment of surface d Channel or pipe diameter coatings or to alter surface states through physical or chemical Q˙ Volumetric flow rate means to actively influence phenomena within microscale and nanoscale systems is also an attractive and growing research area. One measure of growing interest in surface-mediated research 1. Introduction is shown in Fig. 1. This measure can quantify the interest in surface-mediated research by counting the number of citable Surfaces are ubiquitous in our lives. In science and technology, papers published. Two popular scientific databases (Elsevier’s especially microsystems and nanosystems, the role that surfaces Scopus and ISI Thomson’s Web of Science) were used to run play cannot be emphasized enough. Over the years, due to searches on current literature that spans micro- and nanoscale the many technologically important applications in catalysis, systems with surface modification over a 20-year period. Both tribology, electrochemistry, corrosion, and more recently in databases yield quantifiably different results, with the global microsystems and nanosystems surface science has been a subject trends being same for both searches. The Scopus database covers of great scientific interest [1–6]. Given the importance of surface- a wider range of conferences and archival journals as compared mediated phenomena to a wide range of applications, governed to the Web of Science database. Therefore, Scopus shows a much by a few underlying and unifying principles, it is important to higher total number (40,149) compared to Web of Science (16,441) define surfaces. One way to understand a surface (or an interface of published articles as of May 2008. The last twenty years between media) is to consider it mathematically. Mathematically, have seen a continuous increase in the number of papers that a surface can be represented as a plane. However, in most real have been published with 2007 registering over 3500 papers. A world applications a mathematical plane may not fit the definition quick analysis of the subject areas, definitions being somewhat of a surface. In this report, we will define a surface or an interface arbitrary as decided by the databases, shows that most papers to mean a thin region in space (often only a few nm in extent) (∼80%) relate to physics, chemistry, materials science, and biology. within which material properties can vary significantly from the Approximately 16% of all published papers were classified as bulk or remainder of the material. In addition, this thin region of ‘‘engineering’’ papers, suggesting that basic research is the main space influences transport and reaction phenomena in its vicinity. driving force behind recent growth in microscale and nanoscale In this surface-region properties such as chemical composition, systems. Furthermore, it must be noted that the role of surfaces refractive index, mechanical strength, conductivity, charge, etc. with regard to microsystems and nanosystems spans across many can significantly differ from the bulk material. disciplines in both science and engineering. This is evidenced by S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 235 the broad range of literature cited in this report. Therefore, despite process by using an abrasive material such as sandpaper to alter the the authors’ best attempts to be comprehensive; it is likely that surface roughness can be found dating back to ancient times where some references did not find their way into this report. However, hard objects such as small pebbles and shells were attached to bark, many other excellent reviews and books have also been cited animal skin, or paper by using gum resin, creating ancient sandpa- providing the readers with resources to a wide-array of existing per. Modern advances in this simple technology appeared in the literature on the varied subjects of interest. early 1900s with 3M formally patenting and developing the var- In this report, the main focus will be on descriptions of ious coarse and fine-grain sandpapers commercially available to- changing or modifying the properties of a surface through chemical day. Grinding and polishing technologies have now also expanded methods. The chemical methods reviewed here will discuss, for to include methods such as chemical–mechanical polishing or pla- instance, the use of applying surface coatings to alter surface narization [48], which are important to the modern microelectron- energy and surface charge for affecting some phenomena in ics materials processing industry. confined systems. The purpose of this report is to summarize the Another surface treatment and modification method is that role of surfaces in mediating transport and reaction phenomena of thermal treatments. Temperature gradients and thermal treat- within microsystems and nanosystems, with special emphasis on ments have often been used to change surface roughness [49] and fluidic applications governed by the distinctive role of the electric double layer and the molecular-scale interactions that occur within alter the grain sizes and grain boundaries [24]. In the past few microscale and nanoscale systems. years, thermal methods have been employed to create nanoscale One important consideration for all microscale and nanoscale features, facets, textures [50,51], and nanoparticles [52], on a vari- systems and subsequent phenomena in these systems is the role of ety of material surfaces including ceramics [27,53], metals [54,55], length scales. All these systems are characterized by high SA/V ra- polymers [56,57], and semiconductors [58–60], sometimes in the tios, thereby making the role of surface-species interactions critical presence of adsorbates [61–63]. Thermal treatments in presence to system analysis and performance. Fig. 2 presents a pictorial rep- of common gases such as oxygen or water–vapor can be powerful resentation of different length scales relevant to common materi- tools for modifying existing surfaces through creation of steps or als, physical objects, and various microscale and nanoscale systems inducing other forms of nanostructures. For example, it was shown and phenomena. It can be seen from Fig. 2 that microsystems and recently that thermal processing of crystalline α-Al2O3 surfaces nanosystems span a broad range of materials, phenomenological with orientations (1 1 2 0), (1-1 0 2), and (0 0 0 1) which were ◦ length scales over many orders of magnitude, and a wide-variety heat-treated at 1500 C in Ar/O2 and H2/He/O2 led to step for- of applications bridging many different scientific and technological mation and roughening as quantified through AFM topology im- fields. Furthermore, developing such systems and analyzing subse- ages (Fig. 3), but the effects of step formation and roughening were quent surface-mediated phenomena may require a researcher to much less significant on samples heat-treated at 1000 ◦C in moist cross discipline lines as the areas of research will often be multi- O2 [27]. disciplinary. 2.3. Chemical methods 2. Surface modification methods

2.1. Overview Altering the surface chemistry by wet or dry processes is the most common methodology used for chemical modification of Surface modification methods can be divided in two broad cat- surfaces. The processes are so-named because of the processing egories: physical and chemical methods. The definition of these methods and conditions, as discussed next. Modification schemes broad categories depends on how the process actually affects the are governed by a wide range of parameters including sample type surface. Physical methods, in most cases, do not change the chem- (polymers, metals, ceramics, etc.), stability to treatment conditions ical composition of the surface. These methods may change the (for example, thermal or structural), and eventual applications. For surface roughness [22,23], grain sizes and grain boundaries [24, example, polymeric surfaces are often modified by photochemical 25], and faceting [26–28]. Physical methods often relate to use of methods [45,64–72] of which UV irradiation in air [73], other lasers [29], plasmas [30,31], temperature [32], ion beams [33], ball- reactive atmospheres such as ozone [74,75], combined with lasers milling [34,35], and polishing and grinding [32] to alter the surface or lamps [76,77], and grafting surface layers [78,79] is fairly state of a material of interest. While, the main intent with physi- common. The most common mechanism for surface modification cal modification methods is to not alter the chemical composition of polymeric substrates is due to photo-initiated cross-linking of the material, in some cases, physical surface modification meth- or bond scission on these polymeric surfaces. Efficacy of UV ods can lead to changes in the chemical composition of the sur- irradiation is primarily governed by the ability of a material face due to removal or addition of material or chemical reactions to absorb UV light at a given wavelength, and the depth of on surfaces, for example, as in the case of selective or ion-beam modification is determined by the extinction coefficient of the sputtering [36,37] or by selective cross-linking in the presence of material with the intensity variation given by the Beer–Lambert plasmas [38]. law [80]. It should be noted that the Beer–Lambert law has Chemical methods are often classified as such because these limitations, most commonly influenced by self-absorption within methods introduce a change in the eventual chemistry or chemical the material of interest, scattering sites, and deviations due to composition at the surface of a material. The surface may pos- sess chemical properties that are different from the bulk mate- dilution effects. rial. Amongst the methods for chemical modification formation of Amongst the dry surface modification methods use of reactive surface layers [39–41], either covalently bonded or physisorbed, plasmas [30] has been gaining popularity, most likely due the has been most common. Other chemical methods include treat- wide compatibility of materials and integration to microfabrication ment with UV light [42–45] and reactive plasmas [30,31,46,47]. processes for device development. Many different types of The chemical changes to a surface can also introduce a change in gas-plasmas have been cited in literature including air [81], the eventual surface charge density or the surface energy. oxygen [82], H2O[83–88], ammonia [89], and argon [90] for modification of polymer surfaces. Plasma modification methods 2.2. Physical methods have their own sets of advantages and disadvantages. The biggest advantage is probably that surfaces are modified uniformly and One of the oldest methods of modifying the physical character- the modification is limited to a few nanometers in depth without istics of surfaces is by polishing or grinding the surface. A simple affecting the bulk material. The main disadvantage is probably the 236 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254

Fig. 2. A description of length scales as they relate to common materials, physical objects, phenomena, and techniques relevant to microscale and nanoscale systems. Figure based on the work of [10,11,329]. use of vacuum equipment with system parameters that can vary and surface-grafting of various chemically functional polymeric over different systems adding to cost of operation and developing layers [105,113,114]. optimal recipes for surface modification. A detailed discussion Wet-chemical methods often rely on the principles of organic of the various applications and advantages and disadvantages or inorganic chemistry for formation new surface chemical can be found, for example, in the existing literature [30,91–102]. functionalities [39,115]. Consider the example of a polymeric Plasma modification processes generate new chemical species material such as PMMA, which primarily consists of methyl ester on polymer surfaces. The new chemical species can arise due groups on the surface. These methyl ester groups can undergo to surface reactions with reactive gases or due to physical reduction to alcohols with lithium aluminum hydride in ether- sputtering (such as with Ar plasma) caused by active gas-phase based [116] solutions. Alternatively, amino functionalities may species [30,103,104]. These new surface chemical species can also be placed on the PMMA surface through aminolysis of the ester provide an anchor for attaching a series of different molecules that groups by treatment with a solution of N-lithiodiaminopropane display different properties from the underlying bulk-polymer. in cyclohexane; this aminated surface could then be reacted In fact, peroxides generated on polymer surfaces have been with a substituted isocyanate [117] for further surface reactions. utilized for radical based graft-polymerization of methacrylates While wet chemistry methods offer a wide range of possibilities and acrylamides [105]. Furthermore, preferential hydroxylation for tailoring surface properties, these methods also require of poly(methyl methacrylate) or PMMA surfaces by use of a an extensive knowledge of process chemistries for safe and water–vapor plasma as opposed to an oxygen plasma has been reliable methods. Therefore, wet-chemistry methods do not lend used to activate surfaces towards trichlorosilane modification and themselves to easy development across research disciplines. subsequent ‘click’ chemistries to form surface scaffolds of desired functionalities [85,106,107]. 3. Thermodynamics of surfaces Use of reactive gases in presence of photons through UV light and other light sources (e.g., lasers) has also been sometimes 3.1. Overview used to initiate radical polymerization on surfaces for formation of adherent polymer layers. These photochemical methods have Thermodynamics of surfaces and interfaces has been discussed at times been clubbed with plasma methods [30]. Photochemical in scientific literature for a long time, beginning with the work of surface modification has found varied applications including Gibbs in 1878 [118]. Gibbs first presented the idea of a dividing UV-hardening [108], microfabrication and nanofabrication [109– plane to separate the ‘‘bulk’’ from the ‘‘surface’’. The dividing plane 111], development of biosensors and biomolecule arrays [112] is a conceptual construct or a theoretical notion that allows one S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 237

1.0 500 1.0 a-Plane a-Plane a-Plane 0.8 400 800 0.8 2 600 400 200 0.6 1 300 0.6

m 200 m m µ m µ µ 0 0 0 m µ µ 0.4 200 m 0.4 -1 -200 µ -400 -200 0.2 -2 100 -600 0.2 -800 0.0 0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0100 200 300 400 500 0.00.2 0.4 0.6 0.8 1.0 µ m µm µm

1.0 1.0 1.0

0.8 r-Plane 0.8 r-Plane 0.8 r-Plane 400 400 0.6 200 0.6 200 200 0.6 m m m µ m µ m µ 0 m 0 0 µ µ 0.4 µ 0.4 0.4 -200 -200 -400 -200 0.2 -400 0.2 0.2

0.0 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.00.2 0.4 0.6 0.8 1.0 µ µ m m µm

1.0 1.0 1.0 C-Plane C-Plane C-Plane 0.8 0.8 1.0 0.8 1.0 0.6 0.5 500 0.5 0.6 0.6 m m m µ m m µ µ 0.0 m 0 µ

0.0 µ 0.4 µ 0.4 -0.5 0.4 -0.5 -500 0.2 -1.0 0.2 0.2 -1.0 0.0 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.00.2 0.4 0.6 0.8 1.0 0.00.2 0.4 0.6 0.8 1.0 µm µ µm m

Fig. 3. Tapping mode AFM images depicting structural changes to a, r, and c planes of α-Al2O3 as function of temperature and atmosphere. Figure from [27]. to choose the location of surface properties with respect to a recent times, self-assembled monolayers (SAMs) [39,41,124] have hypothetical reference, similar to the mathematical notion of a been used as important tools in altering surface properties [125– surface discussed earlier. The surface properties can vary with 131], building a complex array of surface functionalities based respect to the ‘‘bulk’’ hypothetical reference where the properties on SAMs [132–144], and as underlying building blocks for such as energy, entropy, adsorption, etc. remain constant. In nanoscience and nanotechnology [93,126,135,139,145–153]. One recent times, two extensive reviews have presented detailed main distinguishing feature of SAMs in contrast to surfactant analysis and review of thermodynamics of solid surfaces [119, monolayers is that SAMs are characterized by a specific interaction 120]. In the context of this report, it is important to briefly between the molecule and the substrate [154]. For instance, discuss the thermodynamics of surfaces and interfaces as once a this interaction can lead to the formation of a covalent bond as surface is modified it often yields a change in either the surface observed in the formation of silane-based monolayers on glass energy or surface charge or both, which ultimately affects the or glass-like surfaces through the interactions of the silanol or interactions with the surroundings. These interactions can be hydroxyl groups on the surface interacting with the functional better understood by considering the surface thermodynamics. trichloro- or trimethoxysilane moieties in the adsorbate molecules. It is with this intention that a brief discussion of surface thermodynamics and adherent surface layers is presented next. SAMs can be deposited from either a solution or vapor phase, and formation of SAMs on many different materials has been discussed including metals (for example: Au, Ag, Pt, Cu, etc.), 3.2. Surface layers polymers (for example: PMMA, poly(dimethyl siloxane) or PDMS, polyimide or PI, etc.), semiconductors (for example: Si, TiO , Formation of ultra-thin surface layers (∼10 nm thick or 2 less) either through bottom-up approaches like self-assembly etc.), and oxides (Fe2O3, Ta2O5, SiO2, etc.) [39,40,155,156]. In or application of surface coatings through synthetic chemistry either deposition method, the rate of formation of the monolayer on a variety of surfaces has been a major area of study over depends on the kinetics of the adsorption process. In fact, the the years. The main reason for existing and continued interest kinetics are influenced by the transport of the species to the in modifying surfaces through adherent surface layers is due surface, the surface adsorption, and the desorption of adsorbed to the technological interest in preparing relatively defect-free or other species [157]. Of these steps, the slowest step governs surfaces with systematically engineered properties. Among the the overall kinetics. The functional species adsorbs on the surface surface layers, formation of single molecule thick layers or from the bulk phase near the interfacial region and may undergo monolayers have had a special place beginning with the early surface re-organization to minimize the energy and/or entropy. work of Langmuir on protein monolayers [121–123]. In more The simplest form for a first-order adsorption process is given by 238 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 the Langmuir adsorption isotherm longer) for formation of monolayers from thiol solutions [40,145, c 152,165–168]. θ = (1) A thermodynamic discussion of formation of monolayers would + c b be incomplete without presenting an overview of considering the where, c is the concentration of the adsorbing species near the formation of monolayers through adsorption from the gas-phase. surface, θ is the fraction of surface sites occupied, and b is a A second possible approach to studying adsorption phenomena parameter related to the adsorbent–adsorbate interactions along could be to use equations of state in contrast to the kinetics with an additional dependence on the adsorbate. Eq. (1) applies approach presented above. Therefore, equations of state for for the steady-state or equilibrium condition, when the rate formation of monolayers from the gas phase can also be derived. of adsorption is the same as the rate of desorption. A second The equations of state depend on the chemical potentials of assumption that is important for deriving Eq. (1) is that the the surface and adsorbate and have been derived as shown rate of adsorption is proportional to the bulk concentration of previously [169]. the adsorptive and the empty fraction of the surface [157]. A Several investigations have been carried-out to determine the variety of parameters have been studied for the formation of SAMs. mechanisms for formation of covalently bound monolayers on Some of these include solvent type, processing conditions such as surfaces. Three main mechanisms have been identified. In island immersion time, species concentration, deposition temperature, growth, the surface-active species adsorb to surfaces and form a chain length, and substrate type. vertical stack. As the growth process proceeds, these individual As discussed above, several detailed review articles have stacks begin to coalesce to form ‘‘islands’’ which in turn grow discussed the formation, structure, and properties of organic with increasing deposition time to form the surface-layer. The monolayers on a variety of substrates. A brief discussion of these second mechanism described in literature is uniform growth, the is presented next to demonstrate how surface modification by use surface-active molecules randomly adsorb to the surface and with of self-assembled monolayers can be a powerful tool in developing increasing deposition time the molecules begin packing together. engineered surfaces within microsystems and nanosystems for It is likely that the surface re-organization of the molecules to influencing confined phenomena. Generally, a covalently bonded form the monolayer is driven by entropic minimization. Finally, SAM has three parts: (i) a head-group, which binds to the the third mechanism proposed is the random growth model in surface and also makes linkages between adjacent molecules on which the surface-active species randomly lay down on the surface the surface to form a stable covalently linked surface network, and do not reorganize. It is likely that this form of monolayer (ii) a backbone, which is often a hydrocarbon chain, and (iii) formation leads to defects within the monolayer structure. A an end-group, which is the functional end of the SAM and is pictorial representation of these three mechanisms is presented responsible for surface properties and the interaction between the in Fig. 5, which is based on the work of several researchers functionalized surface and the external surroundings. This self- [160,170–175] that have characterized monolayers using multiple assembled monolayer structure is represented in Fig. 4 [41,158] techniques such as AFM [176–179], X-ray reflectivity [170,180, with the mechanisms for formation of the SAMs also depicted 181], and FTIR [85,182]. Given the complex mechanisms of surface- pictorially. One of the outstanding questions in the formation of layer formation and the multitude of parameters and physical silane-based monolayers is the role played by water during the conditions like substrate type, concentration of surface-active surface reactions [159,160]. species, deposition time, and deposition temperature it is not Continuing with the discussion above, one of the main methods surprising that few well-defined protocols exist for forming high- of chemically modifying surfaces is to form monolayers on surfaces quality surface layers. Most researchers develop their own recipes [161–164]. The monolayer formation in turn leads to changes in the chemical composition of the surface in contrast to the to form surface-layers suited for their study. For example, choice bulk leading to an alteration of the surface chemical potential. of substrate also governs the methodology for characterizing The formation of the monolayers from the solution phase can be the modified surfaces. X-ray photoelectron spectroscopy (XPS) is explained by the Langmuir adsorption isotherm, subject to the a common technique used to determine surface compositions. assumptions discussed above, in which the rate controlling step is However, if azido terminated monolayers are formed on silicon adsorption or glass substrates for subsequent ‘click’ modifications [107,183], additional characterization by FTIR maybe required as the azido dθ ka kd layers undergo faster degradation on glass surfaces in contrast to = c(1 − θ) − θ (2) dt N0 N0 silicon substrates and thus make confirmation of successful surface modification harder. The contrasting XPS data for glass and silicon where, θ is the fractional surface coverage, t is the adsorption substrates from the literature is presented in Fig. 6 and shows time, k and k are the adsorption and desorption rate constants a d that the characteristic surface azide double-peak structure is easily respectively, c is the concentration of the adsorbate in the solution detected on silicon substrates but not on glass substrates [183] phase, and N is the surface adsorbate concentration at full 0 due to degradation of the surface layers under the highly energetic coverage [164]. Integrating (2) with respect to time gives X-ray beams. It should be noted here that the predominant k c   k  k   mechanism of degradation for surface-bound azides is not due = a − − a + d θ 1 exp c t . (3) to photolysis but because of exposure to energetic secondary kac + kd N0 ka electrons [184]. It can be seen from Eq. (3) that the steady-state solution presents Once a surface has been modified by the formation of a the case discussed for Eq. (1) above. In practice, dilute solutions monolayer or attachment of a surface coating, the change most of the adsorbate are often used to form defect free monolayers, commonly manifests as an alteration in the surface energy as there which suggests from (2) and (3) that the rate of surface coverage is a direct correlation between the chemical composition of the should be slow for formation of high-quality surface layers. Forma- surface and the free energy [185]. One experimental technique tion of defect-free monolayers relates to allowing sufficient time used to provide a measure of the surface and interfacial free for entropic processes to assist in the formation of dense, defect energies is the measurement of contact angles [186]. The contact free monolayers. In practice for example, the formation of defect- angles can be related to the surface energy by Young’s equation free thiol-based monolayers on gold surfaces uses this approach of allowing long deposition time (sometimes on the order of 24 h or γSV = γLV cos θc + γSL (4) S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 239

Fig. 4. Schematic showing the structure for self-assembled monolayer and the mechanisms thought to be important for formation of the SAM. The importance of surface- bound water is also depicted in the figure. Figure based on the work of [41,154,158,162–168]. where, in Eq. (4) thermodynamic equilibrium is assumed and γSV , absolute temperature. Now, if the ideal solution with only counter- γSL, γLV are the interfacial free energies between the solid–vapor, ions is replaced by an electrolyte then σs must account for changes solid–liquid, and liquid–vapor states respectively, and θc is the in charge densities of co- and counter-ions in the solution adjacent measured contact angle. In most cases, the largest challenge is to to the surface and those in the bulk solution away from the surface, have an accurate measure of γSL. Simplifications to determine γSL " # have been discussed in literature [187,188], and the following form 2 X X σ = 2εε kT ρ − ρ∞ . (8) of Young’s equation can be used in many practical situations. s 0 0,i ,i i i γLV 2 γSV = (1 + cos θc ) . (5) Moving from the idealized surfaces to a confined microscale or 4 nanoscale system, the surface charge can be related to the ions It is useful to note that Eq. (5) applies for a smooth and in solution by the Grahame equation. Consider an electrolyte homogeneous solid surface for which the difference between the containing a mixture of NaCl and HCl in an aqueous solution within surface tension for the bare solid, γS and γSV is negligible [188]. a glass device. In such a case, the modified Grahame equation In most microfluidic and nanofluidic systems, the role of walls is relates σs to the ions in solution critical in determining subsequent surface-mediated phenomena.  eψ  2 = p 0 As discussed above, the high surface-area-to-volume ratio within σs 8εε0kb sinh confined devices makes the role of the device walls extremely 2kbT important. This influence of walls can be explained mathematically  + + −(eψ /kT )
1/2 × ([Na ]∞ + [H ]∞) 2 + e 0 . (9) by the following discussion, which draws upon the work published over the last century by many researchers [1,189–197], identifying It can be noted from (9) that for surfaces such as glass surfaces and developing the physical laws and parameters that influence that have a native negative charge the surface charge density is confined flows and surface-mediated phenomena. Consider an a function of only the counter-ion concentration in solution. In idealized case in which a surface with some charge density, σs, fact, this property has been exploited in many microscale and is in contact with a solution containing only counter-ions with a nanoscale phenomena by manipulating the buffer concentration. charge density, ρs. The surface is assumed to have a potential, ψs. For example, it can be seen from Eq. (9) that by changing the pH + The surface charge density can be related to the surface potential (i.e. [H ]) the surface charge density can be manipulated. However, by the above equations also suggest that the surface charge density can be directly manipulated through chemical modifications of the  dψ  surface and directly changing σs to affect ψs. σs = εε0 (6) dx s Since, confined surfaces directly play a role in surface-mediated phenomena within devices; let us explore the influence of these and the charge density of counter-ions can be related to those in surfaces. Fig. 7 shows the classic electric double-layer (EDL) the adjacent solution, ρ by 0 structure which is largely responsible for governing transport 2 phenomena within the confined spaces of microfluidic and σs ρs = ρ0 + (7) nanofluidic systems. Several discussions have been presented 2εε0kbT in literature for the EDL and their importance in governing where, ε is the dielectric constant of the medium, ε0 is the microscale and nanoscale fluid phenomena [11,190,195,198–206]. permittivity of free space, kb is Boltzmann constant, and T is the The presence of the EDL and the surface charge, when the surface 240 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254

Fig. 5. AFM images and schematic depict the three common mechanisms for formation of self-assembled monolayers. Figure based on the work of [160,170–176,179]. in a fluidic system is in contact with an ionic or electrolyte of a physical surface quantity that correlates to a specific surface solution arises due to two primary reasons: (1) due to ionization state. For example, contact angle measurements provide a measure or dissociation of surface functional groups (e.g., dissociation of of the interfacial force between a liquid drop and the surface of in- surface hydroxyl or carboxylic acid groups) or by adsorption terest. Contact angle data must be analyzed further to extract quan- of ions from the solution. It can be seen from Fig. 7 that titative information on an actual surface state or relevant parame- depending on the type of surface functionalization and the pH ter such as the surface tension or surface energy. On the other hand, of the surrounding media the same surface can acquire different a technique such as x-ray photoelectron spectroscopy (XPS) pro- surface charge density, which ultimately affects the distribution vides direct verification of the chemical composition on the surface of ions in the EDL. The altered ion distribution can subsequently of interest. It must be noted here that most surface characterization affect transport phenomena (see Section 5.3); therefore surface techniques work well with flat substrates or open interface mate- modification provides a fundamental tool to systematically design rials. Characterization of surface states within confined microscale and control surface-mediated transport. and nanoscale devices is a challenging proposition and continues to be an active research area. 4. Surface characterization 4.1. Indirect methods Once a surface has been modified it needs to be characterized in order to (i) confirm successful and desired modification, (ii) quan- Contact angle (CA) measurements provide quantitative data on tify the modified surface states, and (iii) to evaluate the effect of the interfacial energy between a liquid drop and a solid-surface. the modified surface on the interactions with the surroundings. In Once the contact angle is measured the interfacial energies can be this section, a review of existing surface analysis techniques is pre- calculated from the Young–Laplace equation, as described above sented. The choice of surface analysis methods is a function of the in Eqs. (4) and (5). For complete data, the advancing, receding, desired information about the surface state. Surface analysis and and static contact angles must be measured [207,208]. The most probing methods can be divided into two broad categories: (i) indi- common liquid used is water; however, a variety of liquids have rect methods and (ii) direct methods. These methods are so classi- been used to determine interfacial energies for either quantifying fied because data from indirect methods often provides a measure the modified surface state [85,209] or for applications such as S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 241

it can be correlated to the surface charge density, which can be al- tered through surface modifications as described by Eq. (10) Broad single-peak      2εε0kT eζ 2λD eζ σs = sinh + tanh (10) λDe 2kT a 4kT where, e is the elementary charge, λ is the Debye length or the Glass D EDL thickness, and other parameters have been defined previously. A detailed description of all parameters can be found in the nomenclature section. It can be seen from Eq. (10) that surface

394 396 398 400 402 404 406 charge, σs and the ζ are related through a non-linear functional Binding energy / eV relationship. A direct consequence of changing the ζ potential is the effect on electroosmotic velocity in microchannels as discussed below (see Eq. (13)), which can have important implications for improving separation resolutions in microchip electrophoresis (Section 5.3). Furthermore, ζ potential can be used to quantify the isoelectric points for a surface thereby providing a means to Azide double-peak quantify changes in ionizable groups on surfaces [92,219–221]. Despite the usefulness of ζ potential measurements one of the drawbacks is the lack of information on the number density or chemical nature of surface functional (or ionizable groups). This Silicon drawback is due to the inherent measurement technique which is an average measurement over the entire surface. Therefore, direct methods are often needed to quantify the chemical nature and surface coverage on modified or functionalized surfaces. 385 390 395 400 405 410 Binding energy / eV 4.2. Direct methods

Fig. 6. XPS data for surface modification for formation of different chemically As discussed above, direct methods are so named because they functional layers. Figure from [107,183,184]. provide a direct, quantifiable measure of the surface states. Some direct methods are spectroscopic in nature and provide detailed transfer printing [210–212]. Contact angle data has also been structural and chemical information on the surface states. used to obtain a measure of the surface pKa values [213], which The most common direct method used for surface analysis can be critical in many microfluidic and nanofluidic systems is X-ray photoelectron spectroscopy (XPS) [222,223]. Sometimes since the surface pKa determines the dissociation of the surface- XPS is also called ESCA or electron spectroscopy for chemical bound chemical functionalities and subsequently the electrostatic analysis, though the use of this term is declining. XPS is based interaction between the device walls and the confined electrolyte on the principles of the photoelectric effect. An X-ray beam is solutions. While CA measurements are relatively quick and can used excite photoelectron emission from a small solid-volume near be accomplished with simple equipment, they are prone to the surface (∼10 nm deep). The kinetic energy of the emitted uncertainties due to changes in water quality, temperature, and electrons is measured and related to the binding energy. The relative humidity during measurements [92]. binding energy provides a unique signature for the chemical Surfaces modified with bio-active compounds (see applications composition of the surface. XPS can also be used for quantitative Section 5.4) are often characterized by colorimetric dye assays determination of the relative concentrations of chemical species or by evaluating specific binding events to study the role of in addition to the capability of modern instruments to create surface immobilized biological species. The dye assays are coupled chemical image maps [224–226]. There are two main drawbacks to optical absorbance measurements to quantify the interaction of XPS. First, samples must be handled carefully as the sensitive of the solution-phase dyes with the surface-bound biological nature of the technique makes surface contamination an issue moieties [214–216]. The dye assay methods are plagued with as even small levels of contamination show up in measured spectra. Second, the use of high energy X-ray beams and ultra- problems of sensitive surface detection as the dye interactions high vacuum (UHV) equipment limits the type of samples that with surfaces are determined by electrostatic forces [92]. A more can be used, since soft samples [183,184] or materials likely detailed description of using dye assays and biological activity tests to out-gas pose problems in measurements [85] conducted in for polymer surfaces can be found in a recent review [92]. UHV environment. However, despite these challenges with careful One of the most important surface characterization methods and detailed sample preparation procedures, studies have been with enormous practical implications for microfluidic and nanoflu- conducted on microbial cell surfaces [227]. idic systems relies on use of electrokinetic measurements. As A complimentary technique used with XPS for extracting sur- shown in Fig. 7, at a finite distance from the surface the slip-plane face specific information is Fourier Transform Infrared-Attenuated exists, where a phenomenological quantity the ζ (zeta) potential Total Reflection (FTIR-ATR) spectroscopy. FTIR by itself is most is defined. ζ potential can be measured electrokinetically by either commonly used in the transmission mode to identify the chemi- the current monitoring [217] or the slope method [218]. It must cal bonds present in a compound. For most surface analysis in the be noted that the ζ potential is a phenomenological quantity that transmission mode, IR beams can penetrate to several mm within relates to the surface charge density. However, the concept of ζ the substrate and therefore the technique is not very surface sen- potential can be ill-defined in nanoscale systems with interacting sitive. However, by using a high refractive index crystal and op- EDLs since an explicit slip-plane does not exist within the physical erating in the ATR mode, penetration of the IR beams can be lim- device. In such cases directly using surface charge or surface po- ited to a few µm and surface states can be probed with appropri- tential might be a better way to characterize the surface state [11]. ate background subtraction [228,229]. Many different types of ATR However, for microscale systems ζ potential is a useful concept as crystals are available including ZnSe, diamond, and Ge [92]. Each 242 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254

Zeta

Fig. 7. Schematic depicting the classic double-layer structure with the locations where key potentials are measured. In addition, the influence of surface modification is also shown for a substrate with adherent surface layers. Figure based on the work of [11,107,190–192,196,206]. crystal has a specific wavelength window for IR beam transmis- In addition to the spectroscopic methods discussed above, there sion and therefore the resolution is limited by choice of the ATR are microscopy techniques that can be used to quantify surface crystal. The main advantage of FTIR-ATR over XPS is that equip- states. For example, atomic force microscopy (AFM) has been used ment does not require UHV environments and so a much broader to measure surface charge, surface energy, surface roughness, and range of samples can be used. FTIR-ATR has been used to quantify can also be used to measure slip-lengths [234,235]. The measure- chemical bonding states and surface coverage of monolayers on sil- ment of slip-lengths can be a useful parameter for the develop- ica and glass surfaces [107], structure of water at surfaces in pres- ment nanofluidic systems in order to relate the confined transport ence of an electric field [230] and influence of surface defects [231]. phenomena to the actual surface state [234]. An AFM is a versa- Other review articles describe the theory and use of IR techniques tile tool that in addition to the topography and quantities men- for surface characterization in greater detail [232,233]. tioned above can be used to obtain information about local ma- Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is terial properties such as hardness, elasticity, Hamaker constants, used to determine the chemical functionality and coverage or and adhesion forces [236]. These measurements typically require distribution of ionizable species on a surface. A high energy ion the operation of an AFM to measure force–distance curves that re- beam (commonly Ga+ or Au+) is used to irradiate a surface which late the distance between the AFM cantilever (or probe) and the in turn emits secondary ions. These surface-emitted ions can be interaction between the probe tip and the surface of interest. Fur- separated in a mass spectrometer according to the charge to mass thermore, instruments exist that allow direct measurements of the ratio (q/m). Since the q/m ratio is unique for a given atom, ToF- SIMS provides a unique signature for the chemical functionality. surface forces, like the surface force apparatus developed almost In addition, ToF-SIMS can be used to create chemical images of three decades ago [237,238] allowing measurements of force laws a surface in order to identify the coverage density of the surface in liquids with angstrom level resolutions [236]. Given the impor- layers. For example, Fig. 8 shows ToF-SIMS images for a –Br tance of interfacial characteristics and surface forces in influencing functionalized PMMA surface [85]. The chemical contrast in the phenomena within microscale and nanoscale systems, the surface underlying PMMA substrate and a –Br terminated monolayer helps force measurements provide an important tool for quantification create the chemical image. The main advantage of ToF-SIMS over of surface states. XPS is the extremely high sensitivity (on order of ppb). However, Other microscopy techniques such as scanning electron mi- the disadvantages of working with UHV and high energy ion beams croscopy (SEM) and transmission electron microscopy (TEM) can (∼10 keV or higher, 30 keV is not uncommon) are similar to those also be used for direct imaging of surfaces. Most SEMs and TEMs for XPS. can be coupled with electron or X-ray dispersion techniques S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 243

Bromine terminated PMMA: Oxygen, Counts: 2463072, scale: 10 µm Bromine , Counts: 676323, scale: 10 µm

Silicon, Counts: 102822, scale: 10 µm Total Ion, Counts: 11180144, scale: 10 µm

Fig. 8. Time-of-flight SIMS (ToF-SIMS) data for bromo functionalized PMMA surface. The figure suggests a high coverage density for –Br terminated SAM on a PMMA surface. Figure from [85].

(e.g., EDS) to obtain simultaneous information about the chemi- can be exploited for degradation of water-based organics through cal composition of the surface. However, both SEM and TEM also a series of coupled photo-oxidation and photo-reduction reactions require careful sample preparation and UHV equipment [92]. at the photocatalyst surface. Among the photocatalysts available, titania has been a valuable 5. Applications semiconductor oxide material over the years [260,261] with wide- ranging applications such as use as a white coloring pigment Surface science, over the years, has found many applications in due to the high refractive index (∼2.7), as a heterogenous fields as varied as metallurgy [24,54,239–244], textiles and fabric catalyst [264–266] for water purification [16,267–269], and modification [76,245–247], catalysis [248,249], anti-bacterial and photolysis source for hydrogen production [249]. Titania has microbial surfaces [101,250–256] and many others as discussed in three common crystal forms, rutile, anatase, and brookite. The the introduction section above. With the rapid growth and interest anatase and brookite phase can both be converted to rutile in microsystems and nanosystems, this section will focus on a few through high-temperature treatments. In each crystal phase each applications that are relevant to the broader sphere of modern titanium atom has a coordination number of six with respect societal needs as seen through the microcosm of surface-modified to bonding to oxygen [270]. Titania also exhibits some meta- microscale and nanoscale systems. stable phases, most commonly observed under high-pressure conditions. Amongst all the structures of TiO2, anatase form is 5.1. Photocatalysts for organic decontamination the most relevant for photocatalysis. An extensive discussion of TiO2 structure, properties, and applications has been presented in The field of heterogeneous reactions and catalysis is of recent reviews [261,263]. tremendous practical importance and has been under research Titania is commonly used as a photocatalyst for environmental for the past several decades. Many excellent discussions in remediation of organic pollutants under the trade name degussa reviews and books [257–263] have been published. In this report, P25. TiO2 follows the general mechanism of photocatalytic activity heterogeneous reactions and role of photocatalysts is looked at as shown in Fig. 9 for the degradation of organic pollutants. Photo- from the view of their applicability to problems of environmental oxidation is the process during the photocatalytic cycle that is interest, particularly for water disinfection and decontamination responsible for the degradation of organic contaminants in the by degrading organic molecules in the presence of light. presence of water. As shown in Fig. 9, on exposure to photons There are many different photocatalysts and light activation (UV or visible) an electron from the valence band scales the band- systems that are available to us, beginning with the most gap (∼3.2 eV for titania) and reaches the conduction band [263, common system in nature: the sunlight-chlorophyll system. Most 265,271–274]. The hole recombines with the water in contact ∗ photocatalysis works on a relatively simple principle. As shown with the photocatalyst to generate a hydroxyl radical (OH ) [275]. in Fig. 9, photons are used to excite an electron within the This OH∗ oxidizes organics. The oxidation process at completion photocatalyst from the valence to the conduction band. This generates CO2,H2O, and some residual organic acids [273]. In most photon excitation of electrons creates an electron–hole pair which practical applications including water disinfection some cationic 244 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254

Fig. 9. A conceptual diagram depicting the influence of photon excitation on creating electron–hole pairs in the photocatalyst and subsequent oxidation/reduction reactions for degradation of organic compounds. Figure based on the reviews in [261,263,270]. species (Na+, Ca2+, or other metal ions) are also present in the circuit. However, formation of the electron also creates a corre- aqueous media. These cationic species undergo a corresponding sponding hole, which must be filled. In most cases this is ac- photo-reduction on interaction with the O2− that is generated due complished by keeping the dye in contact with a redox couple to the excited electron [273,274]. in an electrolyte. The I−/3I− redox couple is used commonly One of the main challenges with titania is that the band [280]. Dye-based surface modifications also permit broad absorp- gap corresponds to the UV spectrum for electron transfer. Use tion extending into the near infrared enabling a wide harvest- of UV makes the heterogeneous photocatalysis effort equipment ing of the solar spectrum resulting in larger photocurrents and intensive, and hence there is a move towards trying to operate higher efficiency. Early generation devices were made with dyes titania as a photocatalyst under the visible spectrum. Therefore, such as the Ruthenium-polypyridyl-complex photosensitizers, e.g., 0 0 several surface coatings and dye-sensitizations have been used cis-dithiocyanatobis(4,4 -dicarboxy-2,2 -bipyridine)ruthenium(II) to modify TiO2 surfaces for changing the electron transfer (called N3 dye) due to the ability of these dyes to have strong mechanisms. The simplest modification method involves doping absorption in the visible spectrum, relatively long excitation life- with nitrogen to form oxynitrides [272,276,277] for shifting times, and efficient charge transfer due to the metal cores [282, the band gap into the visible spectrum. However, this doping 283]. However, efficiencies were still limited to about 11% along procedure can introduce crystal defects due to formation of Ti–N with expensive and complicated organic synthesis procedures bonds which reduces UV photocatalytic activity [271]. for the development of these dyes. Challenges with use of Ru- Another method being exploited is to create nanoparticle based dyes have spawned active research in several different mixed-oxides which can be used a potentially high-efficiency configurations of nanowires, nanoparticles, nanotubes, and natural system for generation of OH∗ radicals. For example, recent and synthetic dyes that have been developed for enhancing charge work [278] has shown that by grafting Ta2O5 nanoparticles transfer mechanisms, improving light absorption, and eventually onto SiO2 nanoparticles enhanced photocatalytic activity can be enhancing the efficiency of the solar cell [283–287]. However, sev- observed in contrast to using only one kind of nanoparticle. eral questions remain open with regard to specific mechanisms of light absorption and charge transfer as function of organic dye and underlying substrates, development of nanostructures on underly- 5.2. Dye modified surfaces for energy generation ing substrates to minimize charge-diffusion lengths after creating of electron–hole pairs which can directly lead to improved efficien- One of the most important technological challenges facing cies, and better adhesion of the dyes to semiconductor substrates; scientists and researchers world-wide is the harnessing of so- since most dyes tend to be physisorbed to the surfaces leading to lar power for energy applications for broad impact on problems concerns about dye desorption and degradation and subsequently influencing modern societal needs. Dye-sensitized solar cells or limiting the lifetimes of these solar cells [288,289]. Gratzel cells [279] are being investigated for development of high efficiency solar cells [280]. Use of organic dyes for functional- ization or sensitization of solar cells is advantageous as it pro- 5.3. Microfluidic and nanofluidic separation systems vides an avenue for use of wide band-gap semiconductors such as TiO2. Furthermore, these organic dyes can have large absorp- Systems at the microscale and nanoscale have received signifi- tion coefficients due to intramolecular π–π ∗ transitions and there- cant attention in the past two decades beginning with miniaturiza- fore, enhance the overall efficiency of the light absorption process tion of several analytical chemistry techniques [7,9,12,290–292]. that governs charge transfer and the eventual generation of elec- This interest has translated into developing an increasingly com- tron–hole pairs [281]. Fig. 10 shows the working principle of a plex array of fluidic systems with various integrated optical [293– titania-based dye sensitized solar cell. As the schematic shows, 299], electronic [300–302], and mechanical components [303, upon photoexcitation electrons are injected from the dye to the 304] for a broad spectrum of applications including gas sensing titania and eventually flow through a conductor in the external [305], miniature power and heat sources [306–308], proteomics S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 245

Fig. 11. Schematic depicts the main phenomena within a microchannel in the presence of an electric field. Concepts of electromigration, electroosmosis, and electrophoresis are depicted pictorially by transport of cations, anions, and neutral species.

where ε is the electrical permittivity or the dielectric function of the solution and η is the viscosity of the electrolyte. In Eq. (12), we have implicitly assumed that the Helmholtz–Smoluchowski approximation is valid, which means that the electric double layer thickness is very small compared to the characteristic length for transport. For example, for a channel of radius a, the ratio a/λD  1. The main advantage of working within the small double layer limit is the fact that it allows one to obtain the electroosmotic mobility from flow measurements [107,217] for a given applied electric field without an explicit knowledge of the ζ , fluid viscosity, or the dielectric function of the medium, all of which can change significantly within the EDL [327]. In the case of interacting or overlapping EDLs, as might be the case within nanochannels due to channel dimensions being at the same length scale as the EDL thickness, advanced numerical techniques maybe required to better understand the role of interacting double-layers as some of the basic assumptions such as electroneutrality may not hold. However, some researchers have shown that continuum models can be used for many conditions in devices with channels as small as 10 nm without the need for computationally expensive efforts such as those required in molecular dynamics simulations [205, Fig. 10. A conceptual schematic showing main physical concepts for operation of a dye-sensitized solar cell. Figure based on the work of [279,280]. 328]. Recent reviews have discussed the basic governing equations and numerical techniques that can be used in such cases. Thus, the rate and direction of electroosmosis, via the sign of ζ can and genomics [309–311], nanomedicine [312–314], biological as- be controlled by alteration of the surface zeta potential through says [315–319], chemical separations and detection [320–324], chemical modifications. In CE systems, the achievable resolution, and water purification [16,268]. R, is given by the expression There are two main reasons for the development of surface coatings for microsystems and nanosystems: (i) to reduce or 1 √ ∆µ R = N (13) eliminate analyte-wall interactions in microchip and/or capillary 4 µe + µeo electrophoresis (MCE or CE), and (ii) to enhance separation where N is the efficiency, ∆µ is the mobility difference between efficiencies in CE it is essential to mitigate the effects of the ions, and µ is the ion’s electrophoretic mobility [329]. Eq. (13) electroosmotic flow (Fig. 11). The separation rate and resolution e shows that the separation in CE is a strong function of an ion’s that can be achieved within capillary or chip electrophoretic (CE) electrophoretic mobility, which is a function of the charge-to-mass systems for different target compounds are strongly affected by ratio. The most common material used in CE is silica. An amorphous the surface ζ potentials, which need to be (1) uniform to achieve silica surface is characterized by presence of 4–5 silanol groups minimum deviations from ideal plug-like flow, and (2) adjustable per square nanometer of surface area [330], and is often lower for optimum separation of different compounds. A surface with than the theoretically calculated value of 7.8 groups per square a homogeneous surface zeta potential results in a constant bulk nanometer [330,331]. The silanol groups are –OH terminated and liquid velocity at the edge of the diffuse double layer [325] for at neutral pH dissociate to impart the surface a negative surface the confined electrolyte in the channel. This velocity, called the charge. electroosmotic velocity, veo, can be written as The other main reason is to reduce the interaction between the analyte and the device wall and is often achieved by wall v = −µ ∇φ (11) eo eo passivation, especially for biological molecules. A recent review where µeo is the electroosmotic mobility and φ is the applied has discussed details of the surface science important to biological electric potential. µeo is a measure of the electroosmotic flow systems and molecules [332]. The biggest challenge has been within the channel and is affected by the ζ [195,326]. The in dealing with adhesion of biomolecules such as proteins [333, relationship between µeo and the surface zeta potential, ζ , can be 334] through adsorption processes, and termed as non-specific expressed as binding. These non-specific binding events are driven by a variety of forces such as electrostatic attraction, hydrophobic interactions, εζ  and Van der Waals interactions. Different mechanisms of non- µ = (12) eo η specific binding have been proposed, but several open questions 246 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254

robustness to normal flow and processing conditions in CE. In most cases, the capillary or microchip surface has to be activated [85, 366] prior to covalent attachment of a surface layer. The activation is required to generate active reaction sites with the coating material and to enhance the coverage density. Furthermore, active sites reduce the chances of physisorption thereby reducing the defect densities in the subsequent surface layers. Surface activation can be carried out by a variety of means including piranha or RCA-1 cleaning for glass, silica, and silicon substrates [310, 367] and plasma treatments [30]. There are two main modes Fig. 12. Mechanisms of non-specific adsorption as described in the literature [336– 338] are presented in a pictorial representation. It can be seen that the non-specific of attaching permanent surface coatings following the activation adsorption is a multi-step process beginning with diffusion to surface from the bulk process. The first, is the formation of SAMs. For example, glass and eventually terminating with surface diffusion and binding events. surfaces after activation have a high density of silanol groups. Trichloro-, methoxy-, and ethoxy- silane based organic molecules remain [335]. However, there is agreement that the non- readily react with surface silanols to yield SAMs [39,160,368]. The specific binding events occur due to three main steps [336,337], exact mechanism of surface binding is still under debate, with one (i) diffusion of proteins or biomolecules from the solution to main outstanding question being the role of small (on the order the surface, (ii) reversible adsorption on the surface, and (iii) of a few ppm) quantities of surface-bound water [41,160]. The irreversible denaturing of the protein on the surface. This process second method extends the use of SAMs by treating the SAMs is depicted pictorially in Fig. 12. A reaction-diffusion system of as an intermediate layer. Often the SAM is terminated with a equations is typically used to model binding of various complex reactive end-group permitting further surface reactions to yield a molecules such as fibrinogen or albumin [337,338]. As seen in large array of surface functionalities and structures [107,369–374]. Fig. 12, the protein or complex biomolecule after diffusion from The surface layers can be attached via reactions with pre-formed bulk solution and adsorption to the surface can reversibly desorb polymers or via surface grafting reactions [78,79,105,375–378]. back into the solution, or denature irreversibly and bind to the Amongst the varied surface functionalities and structures that surface. It has been shown that for hydrophobic surfaces, the can be generated through formation of permanent surface layers, binding of proteins is entropically driven [335,336]. Furthermore, two types of permanent surface coatings have found a special surface-bound proteins often undergo conformational changes niche. These are hydrophobic coatings often used in surface driven by an entropic gain or surface-driven dehydration [339– passivation [379–382] and hydrophilic coatings [383–386] often 341]. It must be noted here that biomolecules such as proteins used in biological applications to reduce non-specific binding of exhibit ideal, Langmuir isotherms as expressed by Eq. (1) or (3). biomaterials to walls of the devices. However, the Langmuir isotherms neglect any surface diffusion Toward continued development of high efficiency and high res- effects, and statistical thermodynamics based and kinetics-driven olution separation systems, it is essential to develop fluid manipu- models have been offered to explain surface diffusion, denaturing, lation strategies within microfluidic and nanofluidic devices. Most and agglomeration phenomena [342–346]. transport phenomena at the nanoscale are driven either by elec- Two types of surface coatings have been reported and discussed trokinetic flow or surface mediated transport [11]. Pressure driven in literature. The physisorbed layers formed through secondary flows, more common for macroscale systems, are rarely employed adsorptive interactions are referred to as dynamic coatings and for nanoscale systems due to the large driving pressures required, those attached to device walls permanently through covalent as can be seen from the 1/d4 dependence of pressure drop, |∆P|, bonding are referred to as static coatings [330]. Dynamic coatings on channel diameter in the Hagen–Poiseuille equation [387] are easier to apply, often requiring only a simple rinse of the surface of interest with a solution containing some coating agent. 128ηl |∆P| = Q˙ , (14) Dynamic coatings are preferable as they assist in overcoming πd4 common problems of non-uniformity and irreproducibility. One of ˙ the main drawbacks of dynamic coatings is the lack of robustness where η is the viscosity of the fluid, Q the volumetric flow rate, of the surface layers to flow or shear-based phenomena which are and l the length of the channel. For instance, with water the pres- common in many microfluidic and nanofluidic systems. A large sure drop across a 100 µm long channel, 1 nm in diameter for only −18 3 variety of dynamic surface coatings have been used including an attoliter (10 m ) per second incompressible laminar flow surfactants [347–350], charged polymers such as polyamines, would need a pressure drop from (14) greater than 3 GPa, which is polyimines, and polyarginine [351–356], neutral polymers such impractical for any device. Electrokinetic flows can sustain higher as polysaccharides [357,358], poly(vinyl alcohol) [359–361], flow rates through nanometer channels without excessive pres- poly(ethylene oxide) [362,363], and poly(acrylamides) [364,365]. sures. Thus, electrokinetic flows are a preferred method for driv- Each type of dynamic surface coating displays different efficacy in ing fluid and ions through nanochannels. Therefore, manipulating CE depending on the robustness of binding to the substrate, pH, flows by electric fields is common in these devices and systems. In buffer, and analyte type. For example, one advantage of using PVA- recent years, attempts have been made to develop fluidic analogs based surface coatings over polysaccharides or cellulose derivates of solid-state transistors. Recently, it has been demonstrated that is that with respect to glass or silica surface PVA binds much more these fluidic transistor like systems (Fig. 13) developed with two strongly to the walls, and is not easily washed off thereby providing different types of fluidic confinement – a 2-D SiO2 nanochannels a more robust and stable coating. A more detailed compare and with <50 nm height and ∼1 µm width, and another system with contrast description of dynamic surface coatings can be found in a 1-D nanotube with an internal diameter of <100 nm [388–390]. recent review [330]. Evaluating transport across both these fluidic confines shows ion A second type of surface coating that can be applied is the transport is a function of the surface charge [390]. Surface charge static coating. These coatings, as discussed earlier, are covalently in nanofluidic devices is commonly changed by two methods. One bound to the substrate and are therefore considered permanent. is by directly controlling the surface potential and the other is One of the biggest advantages of the static surface coatings over through chemical modification. Both methods have been reported dynamic coatings, that often require regeneration, is the relative in the literature [11,106,107,166,388,391–397]. S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 247

results. For example, it was found that for the case of EDL overlap the ionic conductance of nanochannels reaches a plateau at low concentrations [388] but it has also been shown that EDL overlap may not be required to observe the plateau region (Fig. 14) for ionic conductance of the nanochannels at low ionic concentrations [390]. Therefore, control of ionic transport in nanochannels remains an active area of research. However, these open questions have spurred several interesting theoretical investigations into the fluid mechanics of confined nanoscale transport with local re-circulation, velocity, and concentration gradients being studied in some detail [398–403]. So far, most of the discussion has related to liquid-phase separations. Another microscale and nanoscale system useful in separation systems is based on the principles of gas chromatog- raphy [381,404,405]. Fig. 15 shows an image for a microfabri- cated serpentine separation column for fast separation (<10 s) for a complex mixture of phosphonate and sulfur compound mixture on the pinacolyl methylphosphonic acid treated micro- column [381]. Next generation detectors being developed for microscale gas-chromatography (µ-GC) rely on functionalized nanoparticles [406], oxime-reactions [407], and non-covalently functionalized carbon nanotubes [408] in contrast to the conven- tional flame ionization detectors for enhanced and rapid detection of gas-phase based chemicals.

Fig. 13. Conceptual schematic and image for a fluidic transistor reported in literature. Figure from [388]. 5.4. Biosensor platforms

Biosensors have broad applicability in diverse fields such as There has been some debate in literature about the role of medical diagnostics [409,410], environmental detection [411– EDL overlap for the influence of surface charge with contrasting 414], counter-terrorism [415,416], water purification [417], and

Fig. 14. Conductance of nanochannels plotted as a function of electrolyte concentration. The data shows the dependence of conductance on surface charge at low electrolyte concentration. Data from [388,390]. 248 S. Prakash et al. / Surface Science Reports 64 (2009) 233–254

1

16 8

2 7 3 15 5 9 10 11 4 6 12 13 14

234567

200 µm

Fig. 15. Digital image and SEM image along with data for separation of a 16-compound organic mixture containing sulfur and phosphate molecules. The data shows fast response time for a microscale gas-chromatography system. Figure from [381]. food safety [92]. In most cases, these sensors rely on a specific 447]. Membrane processes are an integral part of developing fresh- interaction between the analyte and a surface-bound bioactive water supplies through filtration and desalination [448–452]. His- species. This specific interaction is then detected through optical torically, membranes have pore or nanocapillary diameters vary- (e.g., fluorescence), electrochemical (e.g., impedance change), or ing from 1 nm to 1 µm [11]. Membrane fouling [453,454] has colorimetric changes. Depending on the specific surface-analyte been a challenge due to biofouling, scaling, and colloid-driven interaction and the detection method chosen, the sensitivity and fouling. Many different strategies have been employed to tackle dynamic range of the biosensor can be determined. this problem by varying membrane chemical composition, sur- The main objective in developing surface-based biosensors face charge, and morphology [45,375,454–458]. Other strategies [418] is to influence the interactions between the biological include growth of hydrophilic polymers such as poly(ethylene analytes in the sensing environment and the sensor surface. oxide) or poly(ethylene glycol) brushes to reduce biofouling of Consequently, biocompatibility of the modified surface can be membranes [135,459–462]. For example, some researchers have focused on developing different surface modification and selective critical as many biological species require specific conditions to layer copolymerization strategies to mitigate the effects of mem- exist and operate successfully. For example, blood coagulation brane fouling that causes a reduction in the output flux of the mem- can be a major problem in implanted biosensors [419] or non- brane [375,463–465]. Fig. 16 shows images and data from such specific adsorption of proteins to biosensors can cause sensor recent efforts. There has also been a recent effort in trying to en- failure or lowering of operation efficiencies. A detailed discussion hance the flux of clean water through membranes [466–468] and of biocompatibility for modified polymeric surfaces can be found understanding the fundamentals of transport phenomena through in a recent review [92]. As discussed above, one of the strategies confined nanoscale spaces [396,403,457,469–475], and develop- to reduce non-specific adsorption is to increase the hydrophilicity ment of nanofluidic membrane sensors [146,476–483]. of the substrate. Poly(ethylene glycol) surface coatings are most commonly used for this purpose [326,386,420–425]. 6. Summary The most common surface modification technique employed for biosensors is to immobilize an enzyme on the surface of an Many scientific advances in surface science have affected a electrode. This enables electrochemical detection as the redox myriad of applications including emerging and rapidly expand- reaction for binding of the analyte to the enzyme can be monitored. ing fields such as microsystems and nanosystems. Surface mod- With growing improvements in modern electronic equipment, ification in such small-scale systems can be a powerful tool due detection limits of ppb range are now easily achievable [11]. Many to the high surface-area-to- volume ratio in these devices provid- different types of biosensors have been developed [426–436]. ing a direct method for manipulating transport and reaction phe- For example, glucose detection for diabetes has been an active nomena within confined spaces by systematically varying surface research area for sometime [437–441] through various schemes properties. A few of these applications with relevance to prob- such as using amine-activations on cellulose acetate to immobilize lems of public health and welfare directly affecting modern societal glucose oxidase [442], use of functionalized carbon nanotubes, gold needs such as organic decontamination, biosensors, energy gener- nanoparticles, and graphite electrodes [443,444], nanowires for ation, and water purification have been discussed in this report. label-free detection to concentration levels of 8 µM. [445], and The role of surfaces in influencing transport and some fundamen- use of silanization to create avenues for covalent attachment of tals of formation of the surface layers were also presented, includ- bioactive species [446]. ing a brief thermodynamic theoretical framework to understand and answer fundamental questions in microscale and nanoscale surface-mediated phenomena. Given the multi-disciplinary nature 5.5. Water purification of microsystems and nanosystems, with applications in chemistry, materials science, physics, environmental, and engineering sys- A major global technological challenge for the next century tems, this report has attempted a delicate balance between a tuto- lies in developing new technologies for water purification [16,268, rial and a critical review of existing literature. Toward this balance, S. Prakash et al. / Surface Science Reports 64 (2009) 233–254 249

Active Layer of Pure RSA (0-16 nm)

RSA Depletion in Support (~40 nm)

RSA Adsorbed in Support (~600 nm)

Support with Asymmetric Porous Structure (~50 µm)

AD

BE

Recent strategies to mitigate membrane fouling and generate high permeability membranes

CF

Fig. 16. Images showing two strategies from the literature [463–465] based on formation of rigid-star amphiphiles or co-polymerization of PAN-PEO for reducing membrane fouling and developing high permeability membranes for water purification. several open questions for both fundamental and applied studies [16] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, M.J. Marinas, A.M. have been identified throughout this report. It is hoped that this Mayes, Nature 452 (2008) 301–310. [17] F. Besenbacher, J.K. Norskov, Science 290 (2000) 1520. report will assist researchers from diverse fields by providing a col- [18] J. Han, H.G. Craighead, Science 288 (2000) 1026–1029. lection of varied references and encourage the next generation of [19] R.F. Service, Science 310 (2005) 1132–1134. surface scientists and engineers to significantly advance the state [20] M.A. Schwarz, P.C. Hauser, Lab on a Chip 1 (2001) 1–6. of knowledge. [21] P.S. Waggoner, H.G. 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