Anal Bioanal Chem (2013) 405:413–422 DOI 10.1007/s00216-012-6489-2 ORIGINAL PAPER Determination of colloidal gold nanoparticle surface areas, concentrations, and sizes through quantitative ligand adsorption Manuel Gadogbe & Siyam M. Ansar & Guoliang He & Willard E. Collier & Jose Rodriguez & Dong Liu & I-Wei Chu & Dongmao Zhang Received: 1 August 2012 /Revised: 1 October 2012 /Accepted: 8 October 2012 /Published online: 24 October 2012 # Springer-Verlag Berlin Heidelberg 2012 Abstract Determination of the true surface areas, concen- aqua regia-digested AuNPs. The experimental results from trations, and particle sizes of gold nanoparticles (AuNPs) is a the MBI adsorption method for a series of commercial col- challenging issue due to the nanoparticle morphological loidal AuNPs with nominal diameters of 10, 30, 50, and irregularity, surface roughness, and size distributions. A 90 nm were compared with those determined using dynamic ligand adsorption-based technique for determining AuNP light scattering, transmission electron microscopy, and local- surface areas in solution is reported. Using a water- ized surface plasmonic resonance methods. The ligand soluble, stable, and highly UV–vis active organothiol, 2- adsorption-based technique is highly reproducible and sim- mercaptobenzimidazole (MBI), as the probe ligand, we dem- ple to implement. It only requires a UV–vis spectrophotom- onstrated that the amount of ligand adsorbed is proportional eter for characterization of in-house-prepared AuNPs. to the AuNP surface area. The equivalent spherical AuNP sizes and concentrations were determined by combining the Keywords Gold nanoparticle (AuNP) . Ligand adsorption . MBI adsorption measurement with Au3+ quantification of Surface area . Particle size Electronic supplementary material The online version of this article (doi:10.1007/s00216-012-6489-2) contains supplementary material, Introduction which is available to authorized users. : : * Colloidal gold nanoparticles (AuNPs) have found applica- M. Gadogbe S. M. Ansar D. Zhang ( ) – Department of Chemistry, Mississippi State University, tions in many areas including catalysis [1 4], drug delivery Mississippi State, [5, 6], imaging [7], and bio-sensing [8, 9] because they Starkville, MS 39762, USA exhibit unique chemical, catalytic, and electromagnetic e-mail: [email protected] properties. These properties depend on the size, shape, and W. E. Collier composition of the AuNPs. Knowledge about AuNP size, Tuskegee University, surface area, and concentration is vital to determine and Tuskegee, AL 36088, USA understand the correlation between AuNP structure and function. For instance, Wang et al. observed that endocyto- J. Rodriguez Mississippi State Chemical Laboratory, Mississippi State, sis of AuNPs, modified with single-stranded DNA for tar- Starkville, MS 39762, USA geting of cancer cells, depends on the size of the : nanoparticles. It was observed that endocytosis decreases G. He D. Liu with increasing size of AuNPs [10]. Alkilany and Murphy Department of Mechanical Engineering, University of Houston, N207 Engineering Building 1, showed that nanoparticle size plays an important role in the Houston, TX 77204-4006, USA rate and extent of cellular uptake [11]. Also, the catalytic activity of AuNPs is critically dependent on particle size I.-W. Chu [12–14]. Department of Chemical Engineering, Mississippi State University, Mississippi State, Current methods for determining AuNP size can be di- Starkville, MS 39762, USA vided into several subcategories. The first is light scattering 414 M. Gadogbe et al. methods that include dynamic light scattering (DLS), static the published TEM images [29–31]. They are polycrys- light scattering (SLS) [15–17], and small-angle X-ray scat- talline containing different crystal facets with junctions, tering [18]. The second category is the microscopy method vertices, and defects [32–36]. These low to subnanom- including atomic force microscopy (AFM), transmission eter surface features are difficult to account for with electron microscopy (TEM) [19], and scanning electron imaging methods such as TEM and SEM, or other microscopy (SEM). The third is the localized surface plas- existing particle sizing methods, but they can have monic resonance (LSPR) method, which takes advantage of significant impact on the nanoparticle surface areas. the fact that the LSPR features of AuNPs depend on the size, Our hypothesis is that more realistic values for the true shape, and concentration of the AuNPs [20–22]. A more AuNP surface areas can be estimated with molecular recent method is electrospray-differential mobility analysis probes that have small cross sections (<1 nm2/molecule, (ES-DMA) that involves the conversion of AuNP suspen- forexample)ontheAuNPs. sion into the gas phase using electrospray ionization. In ES- Reported herein is a ligand adsorption method for deter- DMA, the charged particles are sorted based on their elec- mining the surface areas, concentrations, and sizes of col- trical mobility and the number average diameter is measured loidal AuNPs, using 2-mercaptobenzimidazole (MBI) as the after counting [23–25]. While TEM, SEM, and AFM probe ligand. While ligand adsorption methods have been images allow direct AuNP visualization [19, 26, 27], these used for determining the exposed surface area of AuNPs measurements require tedious sample preparation, sophisti- (e.g., AuNP catalysts supported on solid substrates) cated equipment, and lengthy and often subjective data [37–39], these methods have, to our knowledge, not been analysis to determine particle size and size distribution. applied to AuNPs in solution. A recent report by Elzey et al. Since only a small population of the nanoparticles is probed, showed that the ligand packing density of the organothiol 3- the results are statistically less representative [27]. mercaptopropionic acid (MPA) on AuNPs is size indepen- One key advantage of light scattering (e.g., DLS) and dent for AuNPs of 5, 10, 30, 60, and 100 nm in diameter LSPR-based methods is their ability to probe the AuNP in [40]. This suggests that ligand adsorption can be a reliable situ, that is, the AuNP in solution, eliminating the tedious method for quantification of the AuNP surface areas and sample preparation required for imaging-based methods. determination of the AuNP sizes. Unfortunately, DLS and SLS suffer from poor robustness Several factors led to choosing MBI as the probe ligand. and accuracy. For example, large uncertainties in particle The first is the high binding affinity of MBI with AuNPs. size analysis can occur in both DLS and SLS induced by Previous research from our group revealed that MBI binds variations in the viscosity and refractive index of the sol- bidentately with AuNPs at neutral pH with a binding con- vent, particulate contaminants in the sample, and fluctua- stant of 4.44±(1.29)×106 M−1 [41]. The high binding affin- tions in the solution temperature. Although DLS proves to ity of MBI is important to ensure that saturation ligand be a useful tool for monitoring slow aggregation processes, binding on the AuNPs can be achieved without using a large Khlebtsov et al. reported that false peaks appear in the size excess of MBI, thereby reducing the cost of chemical range of 5–10 nm in DLS measurements of colloidal AuNPs reagents and minimizing the potential environmental impact [17]. This was attributed to rotational diffusion of nonspher- of this experimental scheme. Using this Langmuir binding ical particles with sizes greater than 30–40 nm [17]. A constant, it can be shown that a 95 % monolayer MBI critical limitation of the LSPR method is its insensitivity to packing can be achieved when the excess MBI is as low small changes in particle size, which is particularly prob- as 5 μM in solution [41]. Secondly, MBI has excellent lematic for AuNPs smaller than 35 nm in diameter [28]. solubility and stability in water. MBI solutions of up to Theoretical calculations have shown that the change in the ≈1 mM can be readily prepared in water, and there is no peak LSPR wavelength is less than 5 nm when the particle detectable change in UV–vis spectra after 8 months of size changes from 5 to 35 nm in diameter [28]. Because of sample storage (data not shown). Thirdly, the amount of their limited reliability, light scattering and LSPR methods MBI adsorbed can be readily quantified by UV–vis spec- are better suited to study gross changes in AuNP sizes troscopy, one of the most reliable and commonly available induced by AuNP aggregation and protein/AuNP binding analytical techniques. MBI has a relatively strong UV–vis than to determine the exact particle size. Although ES-DMA absorbance at 300 nm with a molar absorptivity of can measure very small particle sizes (d<5 nm), the method 27,400 M−1 cm−1. Finally, MBI has a small footprint on requires expensive instrumentation [23]. the AuNP. On the basis of the saturation packing density of Accurate surface area values are crucial when studying 0.574±0.006 nmol/cm2, we recently reported for MBI on the molecular-level ligand interaction with AuNPs. Howev- AuNPs of nominal size of 13 nm by assuming a spherical er, compared to determining particle size, quantifying AuNP shape of the AuNPs [41]; the footprint of MBI on AuNP is surface area is much more challenging. Colloidal AuNPs are 0.29 nm2 per molecule. This small footprint is critical for rarely perfectly spherical or monodispersed, as evident from detecting subnanometer AuNP surface roughness. Determination of colloidal gold nanoparticle