Gold Nanoparticles Modification and Aggregation: Applications from Bio- and Chemosensing to Drug Development

Introduction

The unique physical and chemical properties of noble metal nanoparticles such as gold nanoparticles (AuNPs) open up a substantial field for investigation of new science [1-6]. In comparison with bulk materials, nanoparticles possess a much higher concentration of electrons per surface unit. Therefore, attractive properties of AuNPs result from their coupling to an incident electromagnetic field (i.e., light) that appears as an enhancement of this field [7, 8]. Surface-enhanced Raman spectroscopy [9], fluorometry [10], and localized surface plasmon resonance (LSPR) spectroscopy [11] utilize this feature and have made important contributions to the analytical sciences. A number of practical applications of AuNPs, such as highly effective solar cells [12], cancer therapy [13], integrated optics [14], and chemical and biological sensing [15-17], have also been developed. The LSPR phenomenon depends not only on the wave frequency and structural parameters (shape, size, and chemical nature) of nanoparticles but also on the optical properties of adjacent medium as well as on the distance between nanoparticles. The two latter peculiarities are mainly used

Molecular Plasmonics: Theory and Applications Volodymyr I. Chegel and Andrii M. Lopatynskyi Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-65-5 (Hardcover), 978-0-429-29511-9 (eBook) www.jennystanford.com to develop biological and chemical LSPR sensors based on colloidal AuNPs operating in the surface modification and aggregation modes [18, 19]. The processes of surface modification and aggregation of AuNPs also play a crucial role in novel drug development [20-22].

This chapter presents the results of specific practical studies of colloidal AuNPs interaction with various small molecules and biomolecules and their application for LSPR-based sensing and drug development. The first study deals with the characterization of colloidal AuNPs-biomolecules interaction in the view of a search for optimal LSPR sensor response modes, resulting in an experimental confirmation of theoretically predicted LSPR sensor response enhancement (see Section 2.3.3]. The second study is focused on the experimental and theoretical characterization of small-molecules- induced aggregation of AuNPs, revealing how the differences between and combinations of functional groups in those molecules could promote the aggregation process to achieve a high-sensitive sensor platform. The third study reports on spectrophotometric characterization of interactions in the multicomponent doxorubicin- bovine serum albumin (BSA)-AuNPs system, providing insights on the design of potential prodrugs with regulated properties of antibiotic and protein complexation.

Optical Response of LSPR Sensor Based on Surface Modification of Colloidal Gold Nanoparticles

Mechanisms of LSPR Sensor Response Formation

The optical response of LSPR sensor depends on the structure of its sensitive element and on the sensory mechanism the device is based on. The most commonly used sensor elements of LSPR sensors are based on metal nanoparticles in the form of a colloidal solution or array located on the surface of a solid, and the mechanisms of the formation of the response include aggregation of nanoparticles and modification of their surface under the influence of the analyte. Colloidal solutions ofnoblemetal nanoparticles were thefirstsystems used in early biosensor experiments using the LSPR technique [23- 25] and still remain a current platform for the development of new

LSPR sensors [26-29]. Therefore, the important task is to study the optical response of LSPR sensors based on a colloidal solution of gold nanoparticles upon the surface modification and aggregation of nanoparticles and to find the optimal mode for measuring the LSPR response.

This section presents a description of the LSPR sensor sensitive elements having a structure of a colloidal solution of AuNPs and the results of experimental studies of the influence of surface-modifying analytes on the optical response of this system. Also, the section compares the results of the application of different approaches for the LSPR sensor response estimation (see Section 2.3.3) to the experimental results obtained.

Morphological and Spectral Properties of Colloidal Gold Nanoparticles

One of the most exploited techniques for fabrication of colloidal AuNPs is the well-known Turkevich method [30, 31], which is based on the chemical reduction of gold ions in an aqueous solution under increased temperature. Gold nanoparticles resulting from this technology are almost monodisperse structures with a shape close to the spherical, and about 10 to 15 nm in size (Fig. 8.1a), which are stabilized in the solution by means of weakly bound citrate anions. From the histogram of the nanoparticles size distribution, presented in Fig. 8.1b, it is evident that the average diameter of the nanoparticles in the colloidal solution is about 13 nm. The concentration of AuNPs was estimated to be 8 x 10¹² mL"¹, which corresponds to about 13 nM in terms of molar concentration. This value was obtained taking into account that the entire mass of gold in HAuC1₄ employed for colloidal dispersion preparation was fully transformed into nanoparticles.

Plasmonic properties of the AuNPs colloidal solution are manifested in the form of a peak in the light extinction spectrum, the maximum of which corresponds to a wavelength of 520 nm (Fig. 8.2). The fitting of the experimental extinction spectrum using a theoretical model based on the Mie scattering theory (see Section 2.3.1) showed that the least deviation from the experimental spectrum was observed with an AuNP diameter of 13 nm and a 1-nm-thick citrate-anion coating (Fig. 8.2). When fitted, the size effect of the electron mean free path reduction in AuNPs was taken into account; the citrate-anion layer was interpreted as a saturated monolayer of globular molecules according to Eq. (2.37), where the refractive index of the molecules was set to /i_citrate = 1-575 [33], and water was chosen as the ambient environment. The obtained values of the AuNP diameter and the thickness of the stabilizer layer are consistent with those indicated in the literature [31, 34].

Figure 8.1 (a) Transmission electron microscopy (ТЕМ) imaging of fabricated AuNPs. (b) Histogram of distribution of nanoparticles diameter measurements based on ТЕМ. Inset: High-resolution ТЕМ of single AuNP. Adapted with permission from Ref. [32]. Copyright (2012) American Chemical Society.

Figure 8.2 Measured light extinction spectrum of citrate-stabilized colloidal solution of AuNPs and the results of its fitting using a model based on the Mie theory.

Experimental Study of LSPR Response upon Colloidal Gold Nanoparticles Interaction with Different Analytes

During the interaction of an AuNP in a colloidal solution with analyte molecules, a change in the dielectric properties of the medium surrounding the nanoparticle may occur due to their adsorption and/or the replacement/neutralization of the stabilizer molecules on the surface of the nanoparticle by the molecules of the analyte. Moreover, the investigation ofthe zetapotential upon the interaction of colloidal AuNPs with various analytes showed that depending on the properties of the analyte molecules (in particular, on the magnitude and sign of their charge), the zeta potential may either increase or decrease [32]. This indicates a change in the charge state of the surface of nanoparticles, which can lead to surface modification and aggregation of nanoparticles with the preservation and loss of colloidal stability, respectively.

In order to study the peculiarities of the optical response of the LSPR sensor in the mode of surface modification of nanoparticles, measurements ofthe light extinction spectra were carried out when aqueous solutions of lipoic acid, glutathione, and BSA were added to colloidal AuNPs. Changes in the extinction spectra occurring after the addition of solutions of analytes (Fig. 8.3) indicate a gradual modification of the surface of nanoparticles by molecules of analytes. It should be noted that Fig. 8.3 does not show the initial extinction spectrum of colloidal AuNPs, since as a result of dilution with the addition of solutions of analytes, there is a decrease in the intensity of the extinction peak, which complicates the visual comparison of the initial spectrum with the presented ones. The shifts in the extinction peak position were measured relative to the initial position immediately after the addition of analyte solutions and equaled 1.7 nm, 1.1 nm, and 4.6 nm for lipoic acid, glutathione, and BSA, respectively. The values of the initial peak shifts in the extinction spectra are consistent with the concentrations of analyte solutions (1 mmol/L, 20 pmol/L, and 15 pmol/L) and their molecular weights (206 g/mol, 307 g/mol, and 66,463 g/mol) for lipoic acid, glutathione, and BSA, respectively.

Figure 8.4 shows the measured kinetic dependences of the peak position shift in the light extinction spectrum (LSPR response ДЯ,_пах) in comparison with the kinetic dependences of the LSPR response H_t⁺op corresponding to the spectra in Fig. 8.3, and the results of their fitting by a growing exponential function of type R = R₀ + Ae^A/B. As can be seen from Fig. 8.4, the use of the proposed mode of measuring the LSPR response #_t⁺op on real samples of biomolecules improves the detection capability of LSPR sensor up to 4.5-48 times, depending on the type and concentration of biomolecules, which even exceeds the theoretically derived values (see Section 2.3.3).

Figure 8.3 Measured extinction spectra of citrate-stabilized colloidal solutions of AuNPs after addition of aqueous solutions of (a) lipoic acid (up to a concentration of 1 mmol/L), (b) glutathione (up to a concentration of 20 pmol/L), and (c) BSA (up to a concentration of 15 pmol/L). The arrows indicate the chronological order of measuring the spectra.

Figure 8.5 shows the spectral dependences of the extinction shift in the case of the addition of BSA corresponding to the spectra in Fig. 8.3c. As a base spectrum for their calculation, the spectrum of light extinction measured immediately after the addition ofan analyte was chosen. To determine the spectral position for measuring the LSPR response V_vight, a derivative from the light extinction spectrum of a colloidal solution of AuNPs was calculated prior to the addition of an analyte, and its extremum was found on the right slope of the initial spectrum, whose position was 555 nm.

Figure 8.4 Measured kinetic dependences of LSPR response AA_max compared with kinetic dependences of LSPR response H^_of>, obtained after the addition of aqueous solutions of (a) lipoic acid (up to a concentration of 1 mmol/L), (b) glutathione (up to a concentration of 20 pmol/L), and (c) BSA (up to a concentration of IS pmol/L) to citrate-stabilized colloidal solutions of AuNPs.

Figure 8.5 Measured spectral dependences of extinction shift, obtained after the addition of aqueous solution of BSA (up to a concentration of 15 pmol/L) to citrate-stabilized colloidal solution of AuNPs. Dashed line indicates the spectral position for measuring the LSPR response l/_right; the shaded area indicates the spectral range in which the maximum extinction shift У_тах is produced.

As can be seen from Fig. 8.5, the maximum extinction shift V_max is produced at slightly shorter wavelengths (535-540 nm), and the difference between 7_right and V_max is 10-14%. Thus, the magnitude of response F_right on a real biomolecular sample is close to the maximum value of the "vertical" LSPR response V_max, which confirms the results of theoretical calculations.