Surface Plasmon Resonance for Water Pollutant Detection and Water Process Analysis Peng Zhang 1,2,3, You-Peng Chen 1,*, Wei Wang 4, Yu Shen 1, Jin-Song Guo 1

Trends in Analytical Chemistry 85 (2016) 153–165

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Trends in Analytical Chemistry

journal homepage: www.elsevier.com/locate/trac

Surface plasmon resonance for water pollutant detection and water process analysis Peng Zhang 1,2,3, You-Peng Chen 1,*, Wei Wang 4, Yu Shen 1, Jin-Song Guo 1

1 Key Laboratory of Reservoir Aquatic Environment of CAS, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China 2 Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environments of MOE, Chongqing University, Chongqing 400045, China 3 College of Environment and Resources, Xiangtan University, Xiangtan 411105, China 4 School of Chemistry & Chemical Engineering, Nanjing University, Nanjing 210023, China

ARTICLE INFO ABSTRACT

Keywords: Appearance of various trace and/or emerging pollutants and deteriorated water pollution events chal- Surface plasmon resonance lenge existing analytical methods and water treatment technologies. In the past 15 years, a technology Surface modification Heavy metals termed surface plasmon resonance (SPR) has been widely used for detecting various types of environ- Organic pollutants mental analytes and for improving the water treatment efficiency. In this review, SPR principles and surface Bacteria modification methods are summarized. Applications of SPR in the environmental detection of heavy metals, Microbial attachment organic pollutants and bacteria over the past decade are illustrated. In addition, the use of SPR in moni- Antifouling toring microbial attachment, movement, and biofilm growth and in characterizing antifouling materials is described. While SPR is widely used, there are still considerably more capacities that can be exploited to fully utilize SPR in the investigation of the water pollution control process. © 2016 Elsevier B.V. All rights reserved.

Contents

1. Introduction ...... 154 1.1. SPR ...... 154 1.1.1. SPR principle ...... 154 1.1.2. Localized SPR (LSPR) and long-range SPR (LRSPR) ...... 155 1.2. Ligand immobilization ...... 155 1.2.1. Physical adsorption ...... 155 1.2.2. Thiol binding ...... 155 1.2.3. Covalent immobilization ...... 155 1.2.4. Capture method ...... 155 1.2.5. Polymer film deposition ...... 156 2. Pollutant determination ...... 156 2.1. SPR assay formats ...... 156 2.2. Heavy metals determination ...... 156 2.2.1. Alkanethiol SAM-modified chips ...... 156 2.2.2. Biomacromolecule-modified chip ...... 157 2.2.3. Nanomaterial-enhanced chip ...... 157 2.3. Organic pollutant measurements ...... 157 2.3.1. Pesticides ...... 158 2.3.2. Endocrine disrupting chemicals (EDCs) ...... 158 2.3.3. Polycyclic aromatic hydrocarbons (PAHs) ...... 158 2.4. Counting microorganisms ...... 159 3. Monitoring microbial attachment ...... 159

* Corresponding author. Tel.: +86 23 65935818; Fax: +86 23 65935818. E-mail address: [email protected] (Y.-P. Chen). http://dx.doi.org/10.1016/j.trac.2016.09.003 0165-9936/© 2016 Elsevier B.V. All rights reserved. 154 P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165

4. Probing of antifouling ...... 160 4.1. Inhibiting macromolecules adsorption ...... 161 4.2. Restraining microbial cell attachment and bioﬁlm growth ...... 161 5. Future work needs ...... 162 5.1. Characterizing pollutant removal ...... 162 5.1.1. Biosorbents development ...... 162 5.1.2. Biosorption and metabolism ...... 162 5.2. SPR imaging ...... 163 5.2.1. Observation of microbial motion ...... 163 5.2.2. High-throughput detection of pollutant ...... 163 5.3. SPR combination with other techniques ...... 163 5.3.1. Improving detection level ...... 163 5.3.2. Investigating composition and morphology changes ...... 163 5.3.3. Extending sensing depth ...... 163 6. Conclusions ...... 163 Acknowledgements ...... 163 References ...... 163

1. Introduction a valuable and powerful tool, surface plasmon resonance (SPR), which has been widely used not only for studying environmental pollut- Hazardous materials, such as heavy metals, pharmaceutical ants with a broad range of object dimensions from ions to cells but compounds, and pathogens, are excessively discharged into water also for characterizing the performance of antifouling agents and environment, leading to water pollution and serious threats to probing microbial adhesion and growth events. public health. Many analytical approaches, including UV–visible spectrometry, fluorescence, chromatography, mass spectrometry (MS), 1.1. SPR and atomic absorption spectrometry, have been applied for the detection of environmental pollutants. 1.1.1. SPR principle On the one hand, appearance of various trace and/or emerging The Kretchmann configuration is most commonly adopted to de- pollutants has challenged the existing analytic methods, which typ- scribe the principle of SPR sensing (Fig. 1a). The major components ically can determine only specific types of analytes. On the other of this optical system consist of an excitation light source, a prism hand, the efficiency of water treatment should be improved to reduce and a metallic film (~50 nm) coated on the prism or glass. When energy consumption or even produce energy. For example, anti- an incident light of p-polarization is shined from an optically denser fouling materials can be employed on the membrane surface to medium (glass) into optically thinner medium (gas or water), total decrease fouling and to keep flux in the membrane separation reflection will occur with the incident angle exceeding a critical value. process. Additional, revealing the metabolic pathway for pollutant The incident light produces an evanescent wave that penetrates into degradation is important to enhance the removal via biological the thinner medium with a depth of approximately half of the wave- wastewater treatment. These requirements are excellently met by length, and then returns to the denser medium.

Fig. 1. Principles of SPR sensor and schematic illustration of an SPR sensorgram in a macromolecule adsorption process. (a) A simpliﬁed diagram of Kretchmann conﬁgu- ration for SPR. (b) Changes of SPR angle detected by SPR. (c) A typical monitoring of a macromolecule adsorption onto a surface in real time, and the SPR signal (RU) versus time is recorded during the entire testing process. (I) Exposure of chip surface in buffer and baseline stabilization; (II) adsorption of macromolecule sample onto the chip surface; (III) equilibration of the adsorption and desorption; (IV) surface rinse with buffer. P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165 155

When a thin metallic film exists between the glass layer and the water layer, the free electrons in the metal surface can be excited by the incident light to form surface plasmon wave that propa- gates parallel to the metal surface. SPR is generated when the evanescent wave couples with the surface plasmon wave, which changes the total reflection condition and an attenuated total reflection emerges. As a result, the reflected light energy dramatically decreases due to the transfer of the incident energy to the surface plasmon. A minimum in the reflected light intensity occurs at an angle that corresponds with the incident angle, and this angle is defined as the SPR or resonance angle (θSPR). The SPR angle shifts when a molecular monolayer is formed on the metal surface in the evanescent field at a typical depth of ~200–300 nm. Importantly, this SPR angle shift is ultrasensitive to the refraction index or the mass density of the monolayer close to the metal surface. There- Fig. 2. Common strategies to probe anchoring on gold surface, including (a) phys- fore, a reaction on the chip surface can be monitored in real time. ical adsorption, (b) covalent immobilization (thiol-based and SAMs-based), (c) capture, The SPR angle shift is described in terms of resonance units (RU), and (d) polymer film deposition methods. where an angle shift of 0.1° corresponds to 1000 RU [1]. SPR spectrometry provides an analysis method that is label- free, real-time, rapid and sensitive, and it consumes minimal sample. may be weakly bound, and may have a low packing density. These It has been widely used to acquire the binding specificity between factors may in turn decrease the probes activity [5]. two molecules, a target molecule’s concentration, the kinetic pa- rameters of association and dissociation processes, the binding 1.2.2. Thiol binding affinity, cell adhesion and migration, and so on. Fig. 1c illustrates Ligand with a thiol group (R–SH) can strongly absorb onto the typical SPR data monitoring the dynamic adsorption of macromol- gold chip surface via an Au–S bond, resulting in a homogeneous ori- ecules onto the SPR chip surface in real time [2]. Buffer is flowed entation (Fig. 2b). Ligands that lack the thiol group can use chemical over the chip surface using an appropriate flow rate until the base- synthesis and protein engineering to introduce thiol groups [6]. line stabilizes (Fig. 1c, I). A macromolecule sample is then injected in the flow cell, which then adsorbs onto the chip surface (Fig. 1c, 1.2.3. Covalent immobilization II). The adsorption and desorption processes eventually reach an Self-assembled monolayer (SAM)-based immobilization is the equilibrium state (Fig. 1c, III). Finally, the dissociation process is ini- most studied method to date. A SAM is an ordered molecular struc- tiated by buffer injection to remove unbound or loosely bound ture formed via the adsorption of an active surfactant onto a solid macromolecules (Fig. 1c, IV). The SPR signal (RU) versus time is re- surface in dilute solution [7]. Thiolate monolayers on Au are the most corded during the entire process. extensively studied SAMs on SPR sensor. SAM with different surface properties, such as the wettability, surface charge, and morpholo- 1.1.2. Localized SPR (LSPR) and long-range SPR (LRSPR) gy, can be easily prepared by thiolates with various terminal LSPR is created when incident wavelength matches with the functional groups. SAMs with terminal carboxyl groups can cova- collective oscillations of the electron excited by the incident light lently bind with the primary amine of a ligand based on an amine in noble metal nanostructures [3]. The resonance generates sharp coupling method using activation of the carboxyl with 1-ethyl-3- peaks in the extinction spectrum that are sensitive to the dielec- (3-dimethylaminopropyl)-carbodiimide and N-hydroxysuccinimide; tric of the medium on the surface of the nanostructure. Thus, this approach is widely used to immobilize protein. For carbohy- LSPR can recognize and characterize macromolecules without prism drate attachment, SAMs with terminal carboxyl groups can be configuration. modified with hydrazine or carbohydrazide and activated by Different from the conventional SPR, LRSPR is generated when NaCNBH4; then, the aldehyde group is introduced [8]. The de- a buffer medium is introduced between the prism and the metal- tailed immobilization process of each ligand can refer to the book lic layer, and in which the refractive index is similar to the analyte. [9]. In the covalent immobilization process, the nonspecific binding Compared to conventional SPR, LRSPR sensors have significantly im- of ligand could shroud the reactive functional groups in SAMs, and proved sensitivity and applicability due to their sharper angular decresed the efficiency of immobilization. Furthermore, after the resonance curve, deeper depth of penetration (~1000 nm), and higher ligand immobilization, blocking agents, e.g. ethanolamine is used resolution of refractive index [4]. to block the residual carboxylic groups on the surface to prevent them from interacting with sample. 1.2. Ligand immobilization SAM-based polymerization, such as surface-initiated atom transfer radical polymerization (SI-ATRP) and surface-initiated Ligand immobilization on a gold substrate is normally required photoiniferter-mediated polymerization (SI-PIMP), can graft syn- to recognize a specific receptor, i.e. an environmental pollutant, mac- thesized monomer of a polymer onto the SAM-modified gold surface romolecule or cell, when using SPR chip sensing. Fig. 2 shows the to form a polymer film or brush. For the SI-ATRP, the monomer is common strategies to modify the chip gold surface or directly an- polymerized onto the SAMs as a result of the contribution of CuCl2 choring probes on it. and Tris[2-(dimethylamino)ethyl]amine [10], whereas SI-PIMP can be performed with a monomer solution and photoiniferter SAMs 1.2.1. Physical adsorption via a UV lamp [11]. Physical adsorption is a simple approach, which attaches the target substance on a chip as a result of hydrogen bonding, van der 1.2.4. Capture method Waals forces, electrostatic forces, and hydrophobic interactions The Capture method is an alternative when the ligand activity (Fig. 2a). Hydrophobic head groups or thin layer modification of is damaged or is otherwise unsuitable as a result of formation of a the chip surface can adsorb hydrophobic ligands or its moieties. It covalent method, immobilizing the ligand with a homogeneous ori- is noted that the attachment layer may have a disordered orientation, entation. First, a capture molecule that has a high affinity for the 156 P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165 ligand is covalently immobilized on the chip, and this capture that can specifically bind to the analyte is mixed with various con- molecule subsequently leads to ligand immobilization (Fig. 2c). centrations of analyte in solution, and then each mixture flows onto Streptavidin-biotin [12], antibody-antigen [13], and poly(histidine) the ligand (analyte or analyte derivative). The free macromolecule tagged-nickel-nitrilotriacetic acid [14] capture molecules are the rather than the macromolecule-analyte complex can bind to the commonly used with high affinity in previous studies. However, the ligand. The mixtures are flowed onto the sensor surface in descend- captured ligand will be removed in the chip regeneration process, ing order of the analyte concentration during the test. As the analyte and a new ligand will need to be re-immobilized, which results in concentrations are reduced in the mixtures, the concentration of excess consumption of the ligand. free macromolecules that bind to the chip surface increases, resulting in larger and larger SPR responses. The difference between 1.2.5. Polymer film deposition the two competitive assays is that the macromolecule does not react A highly sensitive SPR signal can be obtained by fabricating a with analyte for the surface competitive assay. The macromolecule polymer nanolayer on the surface of the sensor chip (Fig. 2d). The and the analyte will competitively bind to the ligand immobilized polymer thin film can be formed via chip immersion in a polymer on the sensor. Likewise, the higher the concentration of analyte in solution, a spin-coating method, a Langmuir-Blodgett (LB) tech- the sample is, the lower the response signal is. It is noted that the nique, and other approaches. The spin-coating approach can quickly analyte should have negligible SPR signal in the surface competi- obtain thin layer in several tens of minutes [15]. The LB method can tive assay. Fig. 3 shows the principle and procedure of these SPR better control the thickness and homogeneity of the film, which is assays. usually applied in the preparation of a nanoscale film [16]. Com- pared with the stable covalent immobilization, however, polymer 2.2. Heavy metals determination films are weakly bound to the surface by physisorption [17]. Thus far, the SPR principles and surface modifications were in- Heavy metals are elements with atomic weights from 63.5 to 200.6 troduced in this review. SPR application in the detection of heavy and a specific gravity > 5.0. Zinc, lead, cadmium, mercury, copper, metals, organic pollutants, and bacteria, and its use in monitoring chromium, and nickel are common heavy metals in industrial microbial attachment and antifouling processes were described. wastewater. Conventional techniques of heavy metal ion analysis include atomic absorption spectrometry, atomic fluorescence spec- 2. Pollutant determination trometry, inductively coupled plasma mass spectrometry, anodic stripping voltammetry, X-ray fluorescence spectrometry, and mi- The majority of pollutants in water environments such as heavy croprobes. These existing techniques have several shortcomings; the metal, organic pollutants, and pathogens, can be determined by SPR equipment may be expensive, the dynamic range may be narrow, using an appropriate assay format. This work reviews the use of SPR the samples may be required complicated pretreatment, the tech- in pollutant detection over the past decade. nique may be time-consuming, and so on [18]. Optical sensors are drawing the attention of researchers because of their high poten- 2.1. SPR assay formats tial as an alternative for metal detection. Owing to its proven advantages, such as very high sensitive, portable, inexpensive, fast SPR assays include direct, sandwich and competitive formats measurement capability, simple sample preparation and no neces- (Fig. 3). Macromolecules, viruses, and cells can be analysed by direct sity of reference solution, SPR has been developed for heavy metal format (Fig. 3a). While direct detection for low-molecular-weight determination. Moreover, home-built SPR can also be applied in the analytes does not always show satisfactory SPR signal, this defect quantitative analysis of heavy metal. Fig. 4 shows the schematic il- can be solved using the sandwich and competitive formats. lustrations and principles of the following types of heavy metal The sandwich assay can increase the SPR signal of small mole- detection application. cule in two steps (Fig. 3b). First, the analyte specially bind to the ligand immobilized on the sensor. Second, an amplification-signal 2.2.1. Alkanethiol SAM-modified chips molecule flows over the sensor surface and binds specifically to the Some heavy metal ions can be directly detected by SPR with a analyte, thereby enhancing the SPR signal. gold surface modified by an appropriate alkanethiol. Chah et al. [19] The third format involves solution competitive (inhibition) and treated a Au substrate with 1,6-hexanedithiol to produce SAMs. The surface competitive assays (Fig. 3c). The former performs accord- concentration of divalent mercury in solution was quantified from ing to the following process. A fixed concentration of macromolecule 1.0 nM to 1.0 mM, and divalent mercury presented in mixtures

Fig. 3. Principle and procedure of SPR assays. (a) Direct assay; (b) Sandwich assay; (c) Competitive assay (left, solution competitive; right, surface competitive). P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165 157

Fig. 4. Various types of surface modification for heavy metal detection. (a) Alkanethiol SAMs; (b) Biomacromolecule adsorption or covalent immobilization; (c) Film deposition; (d) Modified nanoparticle binding to the ligand (unbound heavy metal) and enhancing SPR signal. (e) Detected heavy metal as the intermediate between modified nanoparticle and ligand.

containing divalent lead, nickel, zinc, and copper ions was selec- 2.2.3. Nanomaterial-enhanced chip tively measured. Kang et al. [20] developed a selective and recyclable An SPR chip modified by nanomaterials can improve heavy SPR sensor for Cu2+ detection. The gold film was modified by a metal measurements. Wang et al. [28] put forward a method for SAM of 2-aminoethane thiolhydrochloride and then the zwitterion- Hg2+ detection with outstanding sensitivity and a low detection level. like species on the chip surface was neutralized using 1 mM The initial detection limit of Hg2+ was 0.3 μM using the gold film NaOH solution. The concentration of Cu2+ from 100 nM to 1.0 mM modified with a thymine-rich, mercury-specific oligonucleotide was quantified effectively with SPR. The sensor was reusable by re- probe. The detection level was then enhanced and reached 5 nM generation with 1 M HCl for 2 min, which decreases effectively the under amplifying SPR signals using Au nanoparticles modified by determination cost. SPR sensor modified by 6-aminohexane a partially complementary strand sequence. Kim et al. [29] dem- thiolhydrochloride was more sensitive than that modified by onstrated a nanoparticle-enhanced SPR sensing platform for a Ni2+ 2-aminoethane thiolhydrochloride, when the concentration range test. Maleimido-modified N-[5-(3′-maleimidopropylamido)-1- of Cu2+ was above 1.0 mM [21]. carboxypentyl]iminodiacetic acid (NTA) was first immobilized onto an alkanedithiol-modified gold surface, and then the Ni2+ ions adsorbed sequentially onto the NTA modified surface. Next, 2.2.2. Biomacromolecule-modified chip polyhistidine-functionalized quasispherical Au nanoparticles ad- 2+ Forzani et al. [22] developed a method to determine Cu and sorbed specifically onto the surface of Ni2+-NTA complexes and 2+ Ni in the ppt-ppb range using SPR with a surface modification of enhanced the SPR sensitivity, which achieved a detection limit as NH2-Gly-Gly-His-COOH and NH2-(His)6-COOH. The modified surface lowas50ppt. achieved a similar response after regeneration by dipping it in Table 1 summarizes the detection capability for heavy metals 0.1 M perchloric acid for 30 s, which can keep a good capability at by SPR to directly evaluate the different analysis strategies. Accord- least 1 week. Chip modified with mammalian metallothionein mol- ing to Table 1, the concentration of Hg2+,Cu2+,Ni2+,Cd2+,Pb2+, and ecules could sensitively detect Cd, Zn and Ni ions, and it could also Zn2+ in the ranges of 1.0 nM-1.0 mM, 32 pM-1.0 mM, 41 pM-1 nM, selectively detect Cd ion from mixtures of heavy metal ions [23]. 0.13 μM-1 mM, 0.14–24 μM, and 1.52–15.2 μM, respectively, can be Zhang et al. [24] immobilized the metallothionein onto pre- determined using SPR based on currently approaches. Neverthe- formed SAMs and obtained low concentration detection levels of less, Hg2+ detection by 1,6-hexanedithiol SAMs was more convenient, 2+ 2+ Cd (~0.1 μM or 15 ppb) and Hg (5 μM or 1 ppm). Compared with rapid, and ideal in terms of working range than that of other 2+ the result of Chah et al. [19], however, the detection level of Hg methods. Moreover, the detection limit of Ni2+ using nanomaterial- was not improved. enhanced was not improved significantly compared with that using The bare gold surfaces modified by a chitosan film have been NH2-Gly-Gly-His-COOH and NH2-(His)6-COOH. widely used for metal ion detections. Fen et al. [25] produced a thin Compared with other analytical methods, SPR can be used to de- layer of p-tert-butylcalix [4]arene-tetrakis immobilized chitosan film termine the heavy metals in real water samples without sample 2+ on the surface of a gold chip to measure Pb in mixed metal ions, pretreatment. It can directly sense heavy metals in tap water and which detection levels ranged from 30 ppb to 5 ppm. Lokman et al. natural water, which demonstrated it strong capacity in water en- [26] fabricated compound chitosan/grapheneoxide nanostructured vironmental monitoring. Moreover, SPR can also be employed in 2+ thin films to determine Pb ions and obtained high sensitivity and analyzing the water treatment process. The migration process of repeatability at very low concentrations (sub-ppb). Fen et al. [18] heavy metal in the natural water can be simulated by investigat- fabricated a novel active nanolayer with chitosan-tetrabutyl thiuram ing the deposition and release of heavy metal on model surfaces. 2+ disulfide, and the detection sensitivity for Zn was down to 0.1 mg/L. Besides, the kinetics and affinity of heavy metal binding on bacte- Wang et al. [27] used the competitive adsorption of proteins to ria can be studied using SPR, which could promote the removal detect copper ions in drinking water. Albumin samples were de- of heavy metal by biosorption in biological wastewater treatment natured by heavy metal ions through the disruption of ionic system. interactions and disulfide bonds, and these samples were pre- adsorbed on the gold surface of SPR sensor with weak affinity. Native albumin samples with stronger affinity passed through the pre- 2.3. Organic pollutant measurements adsorbed surface and replaced the denatured albumin on the chip based on competitive adsorption, which led to a change in the SPR The main analysis approaches for organic pollutants include UV- angle. The concentration of copper ions below 0.1 mg/L was detected visible spectrophotometry, fluorescence, gas chromatography (GC), using this method. high-performance liquid chromatography (HPLC), MS and GC/LC-MS. 158 P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165

Table 1 The detection capability of heavy metal by SPR with different modiﬁcation strategies

Probe anchored or used Strategies of surface modiﬁcation Detected Detection Dynamic References metals limits ranges

Alkanethiol SAMs 1,6-hexanedithiol SAMs Hg2+ 1.0 nM 1.0 nM-1.0 mM [19] 2-aminoethane thiolhydrochloride SAMs Cu2+ 100 nM 0.1 μM-1 mM [20,21] 6-aminohexane thiolhydrochloride Cu2+ 10 μM >10 μM [21] 2+ 2+ Biomacromolecule-modified NH2-Gly-Gly-His-COOH and NH2-(His)6-COOH immobilized by amino Cu /Ni 32 pM/178 pM — [22] coupling 2+ 2+ NH2-Gly-Gly-His-COOH and NH2-(His)6-COOH adsorbed on gold film Cu /Ni 1.6 nM/41 pM — Mammalian metallothionein immobilized by amino coupling Cd2+ 1 μM 2–10 μM [23] 20–100 μM 200–1000 μM Apo-metallothionein immobilized by amino coupling Cd2+ 0.1 μM 0.13–200 μM [24] Apo-metallothionein immobilized by amino coupling Hg2+ 5 μM 5–500 μM [24] Mercury-specific oligonucleotide SAMs Hg2+ 0.3 μM 0.3–10 μM [28] Chitosan/grapheneoxide nanostructured thin films Pb2+ 0.14 μM 0.14–24 μM [26] p-tert-butylcalix[4]arene-tetrakis coated on chitosan thin film Pb2+ 0.3 μM 0.3–10 μM [25] Chitosan active nanolayer Zn2+ 7.6 μM 7.6–15.2 μM [18] Chitosan-tetrabutyl thiuram disulfide active nanolayers Zn2+ 1.52 μM 1.52–15.2 μM [18] Nanomaterial-enhanced Mercury-specific oligonucleotide SAMs and amplified by gold Hg2+ 5 nM 5–375 nM [28] nanoparticles NTA immobilization and amplified by polyhistidine-functionalized Ni2+ 211 pM 0.2–1 nM [29] quasispherical Au nanoparticles

The sensing approach for organic pollutants detection using SPR in the detection process. In contrast to biomolecules in immuno- biosensors has captured researchers’ interests because of its inher- assays, molecularly imprinted films exhibit more distinct properties ent advantages. in stability, sturdiness, and reproducibility. Dong et al. [39] developed an SPR sensor prepared using a polymerized molecularly 2.3.1. Pesticides imprinted polymer film as the recognition element to detect mala- Pesticides are widely used to kill insects, fungi, and other or- chite green. This approach had high sensitivity (with the analyte ganisms that are harmful to crops. They enter into surface water concentration from 1 × 10−3 to 5 mg/mL) selectivity, and stability. and groundwater in multiple ways, negatively affecting aquatic or- Zhao et al. [40] provided a SPR sensing film based on molecular im- ganisms and human health. printing to detect ametryn, and the modified sensor showed a good SPR immunoassays were widely used to measure pesticide con- linear response at ametryn concentrations ranging from 0.1 to 10 μM. centration. Mauriz et al. [30] used a portable immunosensor based The detection limit of SPR can also be improved by using magnet- on SPR technology to detect carbaryls in natural water samples. The ic molecular imprinting polymer nanoparticles. The SPR chip coated results showed that the method had distinct reusability, reproduc- by magnetic molecular imprinting polymer chlorpyrifos exhibited ibility, and sensitivity with the lowest detection limit as low of an excellent linear interval over the chlorpyrifos concentration range 1.38 μg/L. The accuracy of the SPR results was supported by an HPLC- of 0.001 to 10 μM, and the detection limit was 0.76 nM [41]. MS method. The immunoassay also exhibited highly sensitive measurement of chlorpyrifos (organophosphorus pesticide) in river 2.3.2. Endocrine disrupting chemicals (EDCs) water, drinking water, and ground-water samples, for which the de- EDCs are primarily derived from natural and artificial hormones tection limits were in the range of 45 to 64 ng/L [31]. GC-MS validated and their metabolites, pesticides and their degradation products, the performance of this chlorpyrifos immunoassay. Atrazine in surfactants, plasticizer, some heavy metals, several non-steroidal, natural water samples were determined using an SPR immunosensor food additives, secondary pollutants, and some pharmaceutical and based on polyclonal antibody antisera 11 [32]. The lowest detec- care products. These chemicals interfere with human or animal en- tion concentration was lower than 20 ng/L, and the complete assay docrine systems and cause abnormal effects [42]. time was only 25 min. Likewise, the immunoassay was validated In the same manner as pesticide detection, EDCs can also be using GC-MS. Mauriz et al. [33] detected pyraclostrobin using SPR evaluated by SPR immunoassay. Kim et al. [43] developed a highly immunosensing and achieved detection as low as 0.16 μg/L, and a sensitive and selective SPR immunosensor, and the detection of 2,4- complete assay was carried out in 25 min. Estevez et al. [34] de- dichlorophenoxyacetic acid down to 10 ppt was achieved directly veloped a SPR-based competitive immunoassay for measurement from river water samples. Hegnerova and Homola [44] demon- of thiabendazole. The method was highly specific and sensitive, and strated a measurement of bisphenol A in drinking water, which can the detection limit was 0.13 μg/L under the optimal conditions. Uti- determine the analyte at levels as low as 40 ng/L. At the same time, lizing SPR and immunoassays, the coumaphos concentration was Hegnerova et al. [45] found that the limit of detection for bisphenol quantified, and the sensitivity of this method was low to 25 μg/L A in wastewater was 140 ng/L. Molecularly imprinted SPR is also

[35]. An immunosensor conjugated to Fe3O4 magnetic nanoparticles suitable for EDCs detection. Shaikh et al. [46] developed a water- can enhance the SPR signal. Liu et al. [36] achieved distinct SPR linear compatible molecularly imprinted membrane to determine bisphenol responses for deltamethrin concentrations ranging from 0.01 to A in Milli Q water, drinking water, and synthetic wastewater. Detec- 1 ng/mL and the lowest detection level was 0.01 ng/mL. tion limits for these samples were 20, 60, and 80 ng/L, respectively, Recently, SPR sensor based on molecularly imprinted polymer which were very similar to the results of Hegnerova and Homola [44]. films have been used for pesticide detection. Molecularly imprinted film as sensing materials for specific molecular recognition 2.3.3. Polycyclic aromatic hydrocarbons (PAHs) is widely applied in SPR sensors [37,38]. Molecular targets (analyte) PAHs are hydrocarbon compounds containing more than two act as templates for the polymerization of molecularly imprinted benzene rings, such as pyrene, naphthalene, anthracene and phen- material. Then empty molecular cavities are formed after remov- anthrene. Benzo(a)pyrene is both PAHs and EDCs, and it is the most ing these templates. These cavities can recognize and bind analytes toxic PAH. SPR immunosensors also exhibit excellent properties in P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165 159

PAHs measurements, and the SPR responses were obtained for iron oxide nanoparticles coated with a polysaccharide film were 2-hydroxybiphenyl and benzo(a)pyrene in the concentration ranges modified with a corresponding antibody and amino coupling of 10 ppt to 1000 ppb and 10 ppt to 300 ppb, respectively [47]. Gobi method, and these particles were then mixed with E. coli sample. et al. [48] reported that concentrations of benzo(a)pyrene as low In the LRSPR analysis, the mixture was cyclically flowed on to the as 50 ppt still can be detected by SPR immunosensor. sensor with a magnetic field gradient (∇=0.10 T/mm) penetrating Moreover, the SPR sensors for detection dyes [49], pharmaceu- the chip for 10 min. Then, the flow cell was rinsed, and the sensor tical compounds [50], polychlorinated biphenyls [51], and crude oil chip was regenerated. Torun et al. [61] proposed a similar strategy [52] have been developed. of E. coli detection using SPR combined with an immunomagnetic In summary, the capacity of SPR in organic pollutant was vali- separation, which achieved excellent performance with a detec- dated by GC/LC-MS. Compared with SPR approach, GC/LC-MS tion limit low as to 3 CFU/mL and a linear range from 30 to determine water sample needing to redundant extraction procedure. 3.0 × 104 CFU/mL. The immunomagnetic assay was attained accord- The pesticides, EDCs, and PAHs in tap water, natural water can be ing to the following process. First, an alkanethiol SAM was formed directly determined by SPR. For the turbid water and wastewater on the gold coated magnetic nanoparticles, and then avidin was samples, SPR can also quantify the target composition after 0.45 μm covalently immobilized onto the modified nanoparticles through filtration. It also demonstrated that SPR has strong application po- amino coupling. Next, the decorated nanoparticles were coated with tential in the measurement of organic pollutant in real sample. abiotin-labelled antibody. The antibody-modified nanoparticles were The majorities of SPR assays that detect organic pollutants are mixed with E. coli, and then the complex was determined using SPR. based on immunosensors and molecularly imprinted polymer Moreover, phages immobilized directly on chips can also be films. Indeed, molecularly imprinted polymer film originates from applied to pathogens counts based on the interaction between the immunology. However, the range of available antibodies does not pathogens and the phages, and the detection limits ranged from completely cover existing pollutants in the environment; thus, new ~7 × 102 to 103 CFU/mL [62,63]. antibodies need to be prepared for the future studies. The imperfect Currently, SPR methods for bacterial counts are mainly based on stability and reproducibility of the immobilized antibodies also re- immunoassays; however, their detection limit generally is as high stricts the development of immunosensor assays. Molecularly as ~103 to ~106 CFU/mL (Table 2). Two issues must be addressed for imprinted films have excellent stability, specificity, and reproduc- developing SPR immunoassays for bacterial counts. First, the ex- ibility; however, the preparation technique must also be developed isting immunosensors can only respond to a few bacterial species, and optimized. and thus more sensors are required to extend the number of detectable species. Second, an immunosensor can only recognize a 2.4. Counting microorganisms single bacterial species, which is ineffective for quantifying the total bacteria count in the water environment. Microorganisms, mainly pathogens and opportunistic pathogens, in drinking water distribution systems and other water 3. Monitoring microbial attachment environments may increase potential human health risks. Tradi- tional bacterial counts include heterotrophic plate counts, flow Bio-adhesion, including macromolecule adhesion and cell at- cytometric bacterial cell counts, and epifluorescence microscopy. tachment, is a biochemistry phenomenon that is widely present in Compared with these methods, SPR provides fast and label-free bac- water environments. In fact, bio-adhesion must be enhanced in bi- terial counts. ological wastewater treatment systems to promote microbial SPR chips modified by macromolecules ligands were developed aggregation and biofilm formation to maintain stable reactor op- to detect bacteria. Singh et al. [53] coated the tailspike proteins eration. The process of biofilm formation is described by five stages: derived from the P22 bacteriophage onto the gold surface for (1) planktonic, (2) microbial attachment, (3) microcolony forma- Salmonella enterica serovar Typhimurium detection, and a sensitivity tion, (4) macrocolony formation, and (5) biofilm or cell dispersal limit of 103 CFU/mL was obtained. Yazgan et al. [54] synthesized and [64]. The second stage, the initial attachment of the microorgan- coated carbohydrate ligands, include p-thiolphenyl aminomannose, isms, is a crucial process in biofilm formation. Microbial cell p-carboxyphenyl aminomannose, 1-deoxy-1-aminomannopyranoside, attachment onto a surface is affected by the solution chemistry and glucosamine and chitosan, onto chip surfaces. Their ligands ob- the surface properties of the cell and the substratum [65]. Further- tained high selectivity and ultra-sensitivity for Escherichia coli with more, extracellular polymeric substances (EPS) coated onto the cell a detection level lower than 3 CFU/mL. surface change the surface properties of microbial cells and aggre- A more common method for counting pathogens was based on gates, which generally promote cell deposition, aggregation, and antibody modified chips. Oh et al. [55] immobilized antibodies on biofilm formation [66]. The process of microbial cell attachment and SPR chips in a protein G-mediated manner and achieved the EPS adsorption onto different imitative surfaces can be deter- detection of four types of pathogen, including E. coli O157:H7, mined using SPR because of its sensitive features. Salmonella typhimurium, Yersinia enterocolitica, and Legionella In the past decade, quite a few studies on EPS or microbial cell pneumophila at a level of ~105 CFU/ml. Using eight-channel SPR and adsorption or attachment onto model organic surfaces were carried a sandwich assay, Taylor et al. [56] simultaneously detected E. coli, out using SPR. Pranzetti et al. [67] employed SPR spectroscopy to Salmonella choleraesuis serotype typhimurium, Campylobacter jejuni, investigate in real-time the initial stages of bacterial attachment and Listeria monocytogenes in PBS buffer, and their detection limits (Marinobacter hydrocarbonoclasticus and Cobetia marina) to differ- were 1.4 × 104, 4.4 × 104,1.1× 105, and 3.5 × 103 CFU/mL, respective- ent SAM surfaces. Their results indicated that bacteria readily and ly. A carboxymethylated chip coated monoclonal antibody was used firmly attached to both positively and negatively charged surfaces, to directly quantify Acidovorax avenae subsp. citrulli with a minimum whereas hydrophobic surface restrained bacterial attachment. Based detection level of 1.6 × 106 CFU/mL [57]. These studies demon- on this, Pranzetti et al. [68] developed in real-time a passage between strated that the quantitative levels ranged from ~103 to ~106 CFU/mL reversible and non-reversible bacterial attachment with electro- using chips modified by monoclonal or polyclonal antibodies [58,59]. chemical responsive SAMs. Cell adhesion was promoted by controlling Gold nanoparticles immobilization on the chip surface can also the preferential exposure of charged functional groups (straight chains improve the detection level [59]. Wang et al. [60] achieved a de- with carboxylate anions exposed) on the surface. Guo et al. [69] also tection limit for E. coli O157:H7 as low as 50 CFU/mL based on an prepared four SAMs with different surface properties to evaluate assay combining LRSPR with magnetic nanoparticles. The magnetic the kinetics of bacterial attachment (Bacillus subtilis) and EPS 160 P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165

Table 2 Bacterial counts using SPR sensor with different modiﬁcation strategies

Modiﬁcation strategies Detected bacteria Detection limits Working ranges References (CFU/mL) (CFU/mL)

Biomacromolecule-modiﬁed Tailspike proteins S. enteric serovar Typhimurium 103 103–109 [53] (non antibody) p-thiolphenyl aminomannose E. coli 1.25 × 103 1.25 × 103–1.25 × 107 [54] p-carboxyphenyl aminomannose E. coli 17 17-1.7 × 105 1-deoxy-1-aminomannopyranoside E. coli 2.5 2.5-1.25 × 105 Antibody-modiﬁed Monoclonal antibody E. coli O157:H7, ~105 — [55] S. typhimurium ~105 — L. pneumophila ~105 — Y. enterocolitica ~105 — Biotinylated polyclonal rabbit antibody E. coli O157:H7 1.4 × 104 1.4 × 104–107 [56] S. choleraesuis 4.4 × 104 4.4 × 104–107 C. jejuni 1.1 × 105 1.1 × 105–107 L. monocytogenes 3.5 × 103 3.5 × 103–107 Monoclonal antibody 11E5 A. avenae subsp. citrulli 5 × 105 5 × 105–107 [58] Monoclonal antibody 11E5 A. avenae subsp. citrulli 1.6 × 106 1.6 × 106–109 [57] Polyclonal Immunoglobulin G (IgG) E. coli K12 104 104–105 [59] Polyclonal IgG and gold nanoparticle enhanced E. coli K12 103 103–106 Biotin-labeled rabbit polyclonal antibodys E. coli 35 52–5.2 × 104 [61] immobilized via SAMs E. coli and nanoparticles coated biotin-labeled E. coli 3 30–3.0 × 104 antibody complex Nanomaterial-enhanced T4 phage covalently immobilized E. coli K12 7 × 102 7 × 102–7 × 108 [63] T4 bacteriophage coated E. coli O157:H7 103 103–106 [62] BP14 bacteriophage coated Methicillin-resistant 103 103–105 Staphylococcus aureus

adsorption using SPR. In contrast, these results showed that both imaging methods have been developed, such as intensity, angular, bacteria and EPS tended to deposit onto hydrophobic surface rather phase, wavelength, and polarization, with intensity SPR imaging than hydrophilic surfaces. The inconsistency of these studies could being the extensively used [73]. For the intensity SPR imaging, be associated with the surface tension of cells and the backbone the reflected light that corresponds to the refractive index distri- structures and packing density of the SAMs. For the hydrophilic sur- bution on the sensing surface is captured by a charge coupled device faces, however, there was a consistent result that the charged SAMs camera and translated into a 2-D intensity contrast image (Fig. 5) have more bacterial attachment than neutral SAMs. The effect of the [74]. Abadian et al. [75] applied SPR imaging for the first time to solution chemistry on EPS deposition can be monitored in real-time image the processes of E. coli and Pseudomonas aeruginosa cellular by SPR. EPS adsorption on each SAM could be improved by decreas- movement, attachment, and growth across a large surface. This ing the pH of solution chemistry or by adding mutivalent cations appoach offers important implications for monitoring macromo- [70]. Zwitterionic polymer surfaces exhibit low fouling properties lecular adsorption, microbial cell adhesion and movement, and in resistance of microbial adhesion, including biomacromolecule biofilm formation. adsorption and bacterial attachment [71]. However, divalent These studies suggested that SPR and SPR imaging provide strong cations (Ca2+,Mg2+) can mediate in EPS and zwitterionic polymers support for strengthening microbial attachment and biofilm for- through ion-bridging, thereby enhancing the binding affinity of mation by exploiting water treatment fillers and controlling the EPS and the surface and promoting the bacterial attachment and solution chemistry of the reactor. biofilm formation [72]. Recently, SPR imaging as a novel technique was used to study 4. Probing of antifouling microbial cell physiology in real-time and without labels. SPR imaging can reveal distinctive features of biomaterial, including small Biofilm formation in drinking water distribution system can ac- molecules and cells, at high-spatial resolution on surfaces over a celerate microbial corrosion, worsen overall integrity, reduce residual large imaging area (one square centimetre). Several types of SPR disinfectant levels, release pathogens and other bacteria, and produce

Fig. 5. General principle of SPR imaging [74]. P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165 161 secondary pollutants [76]. In membrane separation processes, that these polymer brushes inhibited protein adsorption, and adsorption and blockage caused by biomacromolecules or fine par- the adsorption of a single protein was less than the limit of ticles and biofilm growth on/in membrane pores can result in the detection (0.3 ng/cm2). Chen et al. [79] prepared poly(N- membrane fouling, which lessens treatment efficiency and mem- acryloylaminoethoxyethanol)-grafted brushes via synthesis and brane performance [77]. Therefore, these adhesive behaviours polymerization of N-acryloylaminoethoxyethanol monomer on gold should be suppressed to decrease harm. Antifouling or nonfouling substrates, and the results showed that they can also effectively sup- surfaces can resist macromolecule adsorption, cell attachment, and press both single protein and undiluted human blood plasma/ biofilm growth. The antifouling abilities of synthetic organic ma- serum adsorption to a nonfouling level (<0.3 ng/cm2). Zhao and Zheng terials have been widely and conveniently examined by SPR. [85] synthesized and grafted poly(N-(2-hydroxyethyl)acrylamide) onto gold surfaces via SI-ATRP, and SPR data demonstrated that 4.1. Inhibiting macromolecules adsorption these polymers can almost completely resist protein (undiluted blood plasma and serum) adsorption at a broad range of film In contrast to other methods used to quantify macromolecular thickness of ~10 to 40 nm. Zhao et al. [10] also demonstrated that adsorption, SPR is the most extensively used in situ. By this method, the polyacrylamide brushes (<0.3 ng/cm2) showed much better re- the reported detection level for protein is below 0.3 ng/cm2 [78]. sistance to proteins (single protein and undiluted human blood Many antifouling polymers were fabricated and characterized using plasma/serum) than the polyacrylate brushes (3.5–7.5 ng/cm2) SPR sensors in previous investigations, and these materials mainly using SPR. focused on hydrophilic polymers and zwitterionic polymers [79]. Zwitterionic polymers, including carboxybetaine, sulfobetaine, Hydrophilic materials, such as oligo(ethylene glycol), poly(eth- and phosphobetaine, exhibit strong hydration induced by electro- ylene glycol), and their derivatives, are the traditional antifouling static interaction and reduce protein adsorption [78]. Liu et al. [11] materials [6]. These materials and other hydrophilic polymers can developed a serine-based zwitterionic poly(serine methacrylate) form hydrogen bonds between the polymers and water to achieve antifouling agent. Poly(serine methacrylate)-grafted films at hydration and reject protein adsorption. Li et al. [80] found that the various thicknesses were synthesized and assessed by SPR. The surface had a stronger antifouling property when it was coated by results showed that the adsorption of bovine serum albumin (BSA) perfluoroalkyl and oligo(ethylene glycol) at a ratio of 37%. Gold sub- (1 mg/ml), undiluted human serum, and human plasma onto the strate modified by hyaluronic acid also showed good protein surface of the optimal film thickness were 1.8, 9.2, and 12.9 ng/cm2, resistance. Using SPR detection, the surface achieved ultralow or low respectively. Thus the protein resistance approached that of the protein adsorptions with 0.6–16.1 ng/cm2 for the model protein conventional antifouling materials. Li et al. [86] reported two amino samples [81]. Gam-Derouich et al. [82] prepared two hydrophilic acid based antifouling zwitterionic materials with different homopolymers for antifouling. The gold chips were modified by thicknesses, poly(N4-(2-methacrylamidoethyl)asparagine) and 4-benzoyl-phenyl moieties originating from the electroreduction poly(N5-(2-methacrylamidoethyl)glutamine). SPR studies showed of the parent salt 4-benzoyl benzene diazonium tetrafluoroborate, that the nonspecific adsorption of undiluted human blood serum and then α-tert-butoxy-ω-vinylbenzyl-polyglycidol (PGL) and and plasma on poly(N4-(2-methacrylamidoethyl)asparagine) hydroxyethyl methacrylate (HEMA) were separately added to two were 0.75 and 5.18 ng/cm2, respectively, and on poly(N5-(2- different modified gold surfaces using by SI-PIMP. The resulting methacrylamidoethyl)glutamine), they were 1.88 and 10.15 ng/cm2, PGL-modified surfaces appeared more hydrophilic than the HEMA- respectively, at polymer film thicknesses of 11–12 nm. Ornithine modified surfaces. SPR tests demonstrated lower amounts of and lysine were used to prepare antifouling zwitterionic materials antibovine serum albumin adsorption onto the PGL-modified surface of poly(ornithine methacrylamide) and poly(lysine methacrylamide), compared with HEMA-modified surface. Dendritic polyglycerol respectively. When the thin films was 14.5 nm, the SPR results functionalized interfaces can be used for preparing protein anti- showed that the adsorption amounts of human blood serum and fouling materials. Paez et al. [83] synthesized dendritic polyglycerol plasma on poly(ornithine methacrylamide) were 1.8 and 3.2 ng/cm2, derivatives bearing disulfide with various amino moieties (0–14%) respectively, whereas those on poly(lysine methacrylamide) were and deposited them onto gold surfaces. SPR measurement re- 3.9 ng/cm2 and 5.4 ng/cm2 from serum and plasma, respectively vealed that surfaces still exhibited excellent protein resistance even [87]. Zou et al. [88] developed a material that had both antifouling at amino contents of up to 9%. and antimicrobial capacities. SPR data illustrated that the protein More outstanding antifouling performance of hydrophilic poly- adsorption on antimicrobial poly(oxonorbornene) dramatically de- mers were prepared and evaluated using SPR. Lin et al. [6] synthesized creased when poly(oxonorbornene)-based poly(zwitterion) grafted two poly(β-peptoid)s with high hydrophilicity, flexible chains, neu- onto their network. This material could be promising challenging trality charges, and strong hydrogen-accepting ability. Poly(N-ethyl- biofilm formation from two fronts. β-alanine) and poly(N-methyl-β-alanine) with end-functionalized Table 3 lists the antifouling properties of the different function- thiol groups were generated through cobalt-catalyzed carbonylative al surfaces characterized by SPR. Both hydrophilic materials and polymerization of N-ethylaziridine and N-methylaziridine, respec- zwitterionic polymers demonstrate excellent antifouling perfor- tively. These groups were grafted onto gold surfaces by Au–S bonds. mance. As described above, however, the antifouling ability of The amounts of protein adsorption onto these two surfaces were zwitterionic surface may decrease when divalent cations are present determined using SPR. The results showed that a single protein sample in the solution. Notably, poly(N-(2-hydroxyethyl)acrylamide), poly((3- adsorbed at levels below the limit of SPR measurement at the pg/mm2 (Methacryloylamino)propyl)dimethyl(3-sulfopropyl)ammonium level, and adsorption of up to ~100 pg/mm2 was detected for full hydroxide) and poly(N-hydroxyethyl acrylamide) surfaces exhibit blood plasma and serum. Pop-Georgievski et al. [84] developed a thorough repression to mixtures of proteins. These polymers can versatile method for preparing antifouling surfaces on different be recommended for use in membrane separation processes to control substrates. Polydopamine films were deposited on gold substrates. membrane fouling and maintain flux. They can also be applied in Subsequently, 2-bromo-2-methylpropanoyl groups were covalently drinking water systems, if they exhibit no biological toxicity. attached to polydopamine films. Polymer brushes of poly(methoxy oligoethylene glycol methacrylate), poly(hydroxy-capped 4.2. Restraining microbial cell attachment and biofilm growth oligoethylene glycol methacrylate), poly(2-hydroxyethyl methacrylate) and poly(carboxybetaine acrylamide) were grafted onto the Microbial cell attachment onto a surface may directly foul that modified gold surface by SI-ATRP. SPR results demonstrated surface and form a biofilm under the appropriate conditions. 162 P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165

Table 3 The antifouling properties of the various functional materials characterized by SPR

Functional materials Resistant targets Maximum antifouling References eﬃciencies (ng/cm2)

Hydrophilic Perﬂuoroalkyl: oligo(ethylene glycol) = 37% BSA (1 mg/L) <5 [80] material Fibrinogen (1 mg/L) <5 IgG (1 mg/L) <5 Hyaluronic acid Lysozyme (1 mg/L) 4.6 [81] BSA (1 mg/L) 7.7 Soybean milk (~10 mg Protein/mL) 0.6 Cow milk (~30 mg protein/mL), 9.8 Undiluted orange juice (~10 mg 16.1 protein/mL) Poly(N-ethyl-β-alanine) and poly(N-methyl-β-alanine) BSA (1 mg/L) 0–1.3 [6] Fibrinogen (1 mg/L) 0–1.0 Lysozyme (1 mg/L) 0.4–1.8 Full blood plasma or serum ~10 Poly(methoxy oligoethylene glycol methacrylate), Human serum albumin (5 mg/L) <0.3 [84] poly(hydroxy-capped oligoethylene glycol methacrylate), Fibrinogen (1 mg/L) <0.3 poly(2-hydroxyethyl methacrylate) and poly(carboxybetaine acrylamide) Undiluted human blood plasma 0–10.9 Poly(N-(2-hydroxyethyl)acrylamide) Undiluted human blood plasma <0.3 [85] and serum Poly((3-(Methacryloylamino)propyl)dimethyl(3-sulfopropyl)ammonium Undiluted human blood serum <0.3 [10] hydroxide) and Poly(N-hydroxyethyl acrylamide) and plasma Zwitterionic Poly(serine methacrylate) BSA (1 mg/ml) 1.8 [11] polymer Undiluted human serum 9.2 Undiluted human plasma 12.9 Poly(N4-(2-methacrylamidoethyl)asparagine) Undiluted human blood serum 0.75 [86] Undiluted human blood plasma 5.18 Poly(N5-(2-methacrylamidoethyl)glutamine) Undiluted human blood serum 1.88 Undiluted human blood plasma 10.15 Poly(ornithine methacrylamide) 100% human blood serum 1.8 [87] 100% human blood plasma 3.2 Poly(lysine methacrylamide) 100% human blood serum 3.9 100% human blood plasma 5.4

Antibacterial methods developed in previous works mainly resist 5.1. Characterizing pollutant removal bacterial attachment and growth. In contrast to hydrophobic or charged hydrophilic SAM, neutral hydrophilic SAM can suppress B. 5.1.1. Biosorbents development subtilis attachment. Pranzetti et al. [68] reported a method of control Adsorption is an effective and economical approach for the M. hydrocarbonoclasticus attachment using conformational changes removal of low concentration organic matters, heavy metals, nitrogen of a SAM occurred at the gold surface. Cell adhesion was inhibited and phosphorus in wastewater treatment. Biosorbent, such as by the controlling preferential exposure of hydrophobic moieties biomacromolecules and microorganisms, is a class of relatively (bent chains with greasy alkyl chains exposed at the surface) on the novel materials in water treatment. However, traditional charac- surface. terization of biosorbent performance involves multiple sampling Abadian et al. [75] monitored the removal process of a mature and measurements, which are time and labor-intensive. SPR can biofilm of E. coli using SPR imaging. Their work illustrated that potentially be applied to conveniently characterize the adsorption biofilm removal was prone to start at some distance upstream and properties, such as the adsorption-desorption kinetics, equilibra- continues from top to bottom. Abadian and Goluch [89] investi- tion time, adsorption affinity, and capacity of a pollutant onto a gated adhesion of P. aeruginosa and Staphylococcus aureus onto casein, biosorbent. As a result, the evaluation of a newly developed biosorbent BSA, and penicillin/streptomycin coated gold surface using SPR using SPR may be preferable. The immobilization of biosorbents imaging. The bacterial cells in the growth medium with a contin- or its active ingredients onto chip using appropriate method would uous flow exposed on these modified surfaces were monitored also need to be addressed. for 24 hours. The results revealed that casein-coated surfaces exhibited good resistance to bacterial attachment over a 24-hour period, and adhesion was reduced by 80% for P. aeruginosa and 5.1.2. Biosorption and metabolism 60% for S. aureus compared to bare gold surfaces. These studies in- Pollutants entering into biological wastewater treatment systems dicated that SPR imaging could be used to study the control of are removed by one of two procedures: biosorption and metabo- biofilm formation and removal. lism by microorganisms. Biosorption occur on the surface of microbial aggregate, scilicet, the surfaces of EPS, and cells. Before investigating the adsorption of pollutants onto EPS or cells, the im- 5. Future work needs mobilization method for EPS or cells onto an SPR chip in a manner that retains their activity must first be researched. The pollutant may According to previous studies, SPR has been widely used in the be absorbed onto the cell surface or enter into the cell as a sub- detection of environmental pollutants and monitoring bio-adhesion. strate that is degraded by enzymatic catalysis. Research on the Still, exploitation of the powerful capabilities of SPR may prove interactions between two molecules is the inherent advantage of worthwhile in the following fields of water pollution process in SPR. Furthermore, SPR analysis requires for limited sample amounts. the near future. Indeed, the reactions of substrate with enzyme immobilized on a P. Zhang et al. / Trends in Analytical Chemistry 85 (2016) 153–165 163 chip have been monitored by SPR [90]. Therefore, to study the key 5.3.3. Extending sensing depth process of enzymatic degradation of a given pollutant by SPR, the The sensing depth of conventional SPR is lower than 300 nm, metabolic pathway of the pollutant should be known. In addition, which can only acquire part of the volume of a cell. The penetra- the protocol for immobilization of the enzyme onto chip requires tion depths of LRSPR and SPR waveguide mode reach as large as study to avoid reducing the enzyme activity. ~1000 nm and several μm, respectively, which must be utilized to obtain relative thick dielectric layers, such as a monolayer of cells 5.2. SPR imaging [98].

5.2.1. Observation of microbial motion 6. Conclusions SPR imaging has been employed to probe cell biochemical reactions, such as observation of interactions between rat cells and SPR provides an analysis method that is label-free, real-time, the extracellular matrix [91] and determination of antibody pro- rapid, and sensitive and minimal sample consumption. For nearly duction by individual hybridoma cells [92]. These applications have a decade, SPR has been largely employed to investigate water pol- paved a way for investigations into microbial motion in biological lution and analyse water treatment process. Most environmental wastewater treatment process. Future work on microbial motion, pollutants, including heavy metals, organic matters, and pathogens, such as quantification of EPS secretion by microbes and explora- can be determined by SPR using the appropriate surface modification of dynamic interactions between emerging pollutants and tion. The lowest detectable concentrations of heavy metals are bacteria, ought to be thoroughly researched using SPR imaging. as low as to pM-μM. Organic pollutants, such as pesticides, EDCs, PAHs, dyes, pharmaceutical compounds, polychlorinated biphe- 5.2.2. High-throughput detection of pollutant nyls, and crude oil, are quantified using immunosensors and SPR analysis exhibits a strong applicability to a broad range of molecularly imprinted polymer film methods. SPR exhibits strong molecular weights or particle sizes. However, conventional SPR has capacity in directly determining the real water sample compared only several channels, which results in a deficiency in analysing traditional methods. In addition, SPR can also count common large-scale samples. Fortunately, high-throughput and rapidly pathogens. analysis has been continuously circumvented with the birth of SPR is inherently superior at monitoring microbial adhesion SPR imaging. Microarrays of various ligands or probes on the same process. To promote bacterial aggregation and biofilm formation to chip can be used to analyse a sample for several pollutants simul- steadily run of wastewater treatment systems, many studies of mi- taneously. The binding of analytes or targets from a solution to a crobial cell/EPS attachment, movement, and growth on model organic microarray would change the SPR image. The current arraying surfaces were performed by SPR and SPR imaging. In contrast, two methods include spotting (lattice array), microfluidic channels (line types of antifouling materials (hydrophilic and zwitterionic poly- array), and on-chip synthesis. Hundreds or thousands of targets mers) have been characterized using SPR. binding on a microarray have been analysed using developed SPR To exploit the full capability of SPR, research focusing on imaging [93]. These indicate that microarrays based on SPR imaging biosorbent development, biosorption and metabolism, observa- should be developed for pollutant detection. Moreover, more efforts tion of microbial activity, high-throughput detection by SPR imaging, should be made to obtain high-throughput analysis for environ- and the combination of SPR with other techniques are worth- mental pollutants, including developing more microarray methods while pursuits that out to be carried out in future studies. and improving their sensitivity, resolution and reusability. Acknowledgements 5.3. SPR combination with other techniques The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (51578527, 51508546 5.3.1. Improving detection level and 51278509), the Public Experiment Center of State Bioindustrial Combination of the traditional electrochemical methods, SPR and Base (Chongqing) and Shanghai Tongji Gao Tingyao Environmen- magnetohydrodynamic convection over the detection range of 1 fM + tal Science and Technology Development Foundation. to 1 μM Hg2 was achieved, which significantly decreased the detection limit for Hg2+ [94]. Thus, to improve the detection limit for pollutants, SPR should be combined with other instrument. References

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