Application of gold nanoparticles in the methods of optical molecular absorption spectroscopy: main effecting factors

Preparation of spherical AuNPs

Preparation of gold nanorods

AuNRs were prepared using a seed mediated approach according to [14]. (1) Preliminarily, a seed solution was prepared: 5 mL of 0.5 mmol L⁻¹ HAuCl₄, 5 mL of 0.20 mol L⁻¹ CTMA and 0.6 mL of ice-cold 0.010 mol L⁻¹ NaBH₄ were added into a round-bottom flask under stirring. Vigorous stirring of the solution was continued for 2 min.

(2) 1.25 mL of 0.0040 mol L⁻¹ AgNO₃, 25 mL of 0.20 mol L⁻¹ CTMA, 25 mL of 1.0 mmol L⁻¹ HAuCl₄ were added into a round-bottom flask under stirring. Then 0.35 mL of 0.0788 mol L⁻¹ ascorbic acid was added dropwise into the solution. Then 60 μL of the seed solution were added. The color of the solution was gradually changed into reddish-brown within 30 min. The temperature of the solution was 50°C during the synthesis.

The obtained solution was kept for 2 weeks, centrifuged for 30 min at 8000 rpm in order to precipitate AuNRs, and the excess of CTAB was removed by decantation of the supernatant. AuNRs were washed in deionized water by redispersion of the sediment at sonication. The procedure was carried out twice. The concentration of AuNRs in the final solution was 71 mg mL⁻¹ (0.36 mmol L⁻¹ in terms of gold).

Preparation of PUF-based nanocomposites

For preparation of PUF-based nanocomposites of various AuNPs, a general approach including adsorption modification of PUF with AuNPs from an aqueous solution under agitation by shaking. This approach has been described earlier [15], [16], [17]. It is quite universal and can be applied to AuNPs of different types.

Charge and nature of a stabilizer

To investigate this aspect, a systematic study and comparison of the processes of AuNPs aggregation under the influence of compounds of various classes was carried out. The aggregation depends on colloid stability of the nanoparticle dispersion, which is usually also governed by general lows, not only those specific for AuNPs [18], [19], [20]. Three types of AuNPs were synthesized and characterized: negatively charged citrate-stabilized AuNPs, positively charged cetyltrimethylammonium-stabilized AuNPs and 6,6-ionene polycation-stabilized AuNPs. The main difference between nanoparticles is their charge and type of stabilizer (Table 1). Besides charge, these three types of stabilizers were chosen considering the following speculations. Citrate and CTMA are low molecular weight substances that are frequently used in preparation of negatively and positively charged AuNPs, respectively. Therefore, a systematic study on their effects on AuNPs analytical features is important for practice. On the contrary, 6,6-ionene is a rarely used stabilizer. However, it is an important example of high molecular weight polycation that can provide additional steric stabilization of AuNPs.

Table 1:

Properties of prepared gold nanoparticles.

Stabilizer	Citrate	CTMA	6,6-Ionene
Formula of a stabilizer
ζ-potential, mV	–19	+41	+8
λSPR, nm	525	515	520
Molar extinction coefficient, L mol−1·cm−1	1.4·109	3.0·106	5.0·108
Hydrodynamic diameter, nm (DLS method)	42	35	42

Compounds of three types were considered as determined compounds (Table 2): thiocompounds, the thiol group of which is capable of forming a strong bond with gold; cationic compounds, which can electrostatically interact with negatively charged nanoparticles; and anions, which are capable of interacting with positively charged nanoparticles. When choosing the compounds, the presence of substituents of various nature and the possibility of existence in solution in different charge states, as well as the relevance of developing methods for the determination of certain compounds were taken into account.

Table 2:

Determined compounds.

Chemical name	Formula	Form of existence in a solution at pH 4–6	Abbre-viation
Thiocompounds
Cysteamine		Cation	CA
Cysteine		Zwitterion	CN
Acetylcysteine		Anion	AC
Glutathione		Anion containing a zwitterionic part	GLU
Mercaptoethanol		Uncharged molecule	ME
Mercaptopropionic acid		Anion	MPA
Cationic compounds
Polyhexamethylene-guanidine hydrochloride		Polycation	PHMG
Neomycin		Multi-charged cation	NEO
Anions
Sulfate	SO42−	Two-charged anion	–
Pyrophosphate	P2O74−	Two-charged anion (dihydro-)	–
Oxalate	C2O42−	Two-charged anion	–
Other	Cl−, Br−, I−, F−, SO32−, S2O32−, ClO3−, NO3−, NO2−, ClO4−, PO43−, CH3COO−, CO32−	Anion	–

The AuNPs aggregation degree was evaluated by the ratio of absorbances of the aggregates and the absorbance at 520 nm (A_agg/A₅₂₀). This parameter was also used as an analytical response during spectrophotometric determination of the compounds. The results of interaction of AuNPs with the compounds are represented in Fig. 1. It can be seen that AuNPs/Cit in an aqueous solution in the absence of additional reagents aggregate only in the presence of cysteamine, neomycin and PHMG. With an increase in the concentration of the compound up to 150 μg mL⁻¹, aggregation by glutathione is also observed. In this case, aggregation is associated with the efficient neutralization of the negative charge of citrate-capped AuNPs by adsorbed cationic species. Also one can consider interparticle electrostatic interaction via positively charged analytes. AuNPs/CTMA having a high value of ζ-potential are very stable and do not aggregate in the presence of compounds of the studied types. This also may be related with high molar concentration of CTMA used in AuNPs/CTMA synthesis. High amounts of CTMA in a solution compete with an analyte on the surface of AuNPs decreasing its aggregative effect. The aggregation of AuNPs/Ion is possible only in the presence of ions that carry a relatively large negative charge: sulfate, pyrophosphate and oxalate. Aggregation in the presence of carbonate, phosphate, and sulfite is not observed, apparently, since they are protonated under experimental conditions, which reduces their charge. It is important to note that the aggregation proceeds only at relatively high amounts of analytes, which can be ascribed to desensitization of AuNPs because of their steric stabilization by 6,6-ionene polymeric chains.

Fig. 1:

Aggregation degree of AuNPs stabilized with citrate (a), CTMA (b) and 6,6-ionene (c) in the presence of thiocompounds, c=1 μg mL⁻¹ (1), cationic compounds, c=1 μg mL⁻¹ (2), and anions, c=1 mg mL⁻¹ (3). * Cl⁻, Br⁻, I⁻, F⁻, SO₃²⁻, S₂O₃²⁻, ClO₃⁻, NO₃⁻, NO₂⁻, ClO₄⁻, PO₄³⁻, CO₃²⁻, CH₃COO⁻.

The aggregation effect can be used to develop methods for determining compounds that exist under certain conditions in the form of multiply charged particles. In addition, thiocompounds and certain anions can be determined in this way. However, sometimes this is possible only in the presence of substances that reduce aggregative stability of AuNPs. E.g. addition of ethylenediaminetetraacetic acid (EDTA) effectively decreases aggregative stability of AuNPs/Cit, simultaneously playing the role of a masking agent preventing interferences from metal cations [12], [21]. Analytical features of merit for the spectrophotometric determination of different compounds using AuNPs are represented in Table 3. The limits of detection (LODs) depend on the nature of the compounds determined and can reach down to 0.01 μg mL⁻¹. The developed methods can be used in the analysis of pharmaceuticals, urine, food additives of disinfectants and water.

State in the analytical system

A remarkable effect on the characteristics of AuNPs as analytical reagents is exerted by their state in the analytical system – whether they are present in free form in a solution or immobilized on surface of a solid support. In our work, polyurethane foam (PUF) is considered as such a support. This polymeric material has been well studied in chemical analysis as a sorbent for preconcentration of substances from water and air [22], [23], [24]. It is known that PUF contains functional groups of different polarity and electronic properties, which favors the immobilization of various types of nanoparticles [15], [16]. In addition, it is an easy-to-use durable and chemically stable monolithic material containing an open pore system.

PUF-based nanocomposites of AuNPs were prepared using the approach proposed earlier [15], [16], [17]. It has been shown that AuNPs are able to aggregate on the surface of PUF under the influence of thiocompounds [15], as can be seen from the TEM images represented in Fig. 2. In this case, a change in the optical characteristics of the samples is observed, which can be used for the determination of the compounds by solid-phase spectroscopic methods, e.g. diffuse reflectance spectroscopy.

Fig. 2:

A TEM image of PUF–AuNPs/Cit nanocomposite before (a) and after (b) interaction with 1 μg mL⁻¹ solution of cysteine.

It should be stressed that selectivity of AuNPs aggregation on the surface of PUF is essentially different compared with an aqueous solution (Fig. 3). Aggregation of AuNPs on PUF is caused by thiocompounds both in the case of AuNPs/Cit and in the case of AuNPs/Ion. This is quite different from the behavior of the same AuNPs in a solution that has been discussed earlier. A particularly strong difference is noticeable with respect to the cationic compounds and anions, which cause aggregation of the corresponding AuNPs in solution and do not cause it on PUF. Aggregation of AuNPs on PUF seems to be associated with some peculiarities. First of all, AuNPs on PUF have closer location to each other. In other words, they are “pre-ordered” to aggregation. This improves sensitivity of the system. Another factor facilitating aggregation is absence of excess of a stabilizer compared to a solution, since the main amount of a stabilizer is not adsorbed by PUF. However, at the same time, AuNPs on PUF are lower mobile. In addition, an important role affecting selectivity can be ascribed to the PUF adsorption properties, since this polymer is unable to absorb highly charged compounds. Thus, it can be concluded that aggregation of AuNPs on PUF depends mainly on the specific interactions, whereas aggregation in an aqueous solution is affected by the charge state of a compound.

Fig. 3:

Comparison of selectivity of AuNPs/Cit (a, b) and AuNPs/Ion (c, d) aggregation in a solution (a, c) and on PUF (b, d) in presence of cysteamine, cysteine, PHMG and sulfate.

Analytical capabilities in some thiocompounds determination by diffuse reflectance spectroscopy using PUF-based nanocomposites are illustrated in Table 4. The methods can achieve low limits of detection. It should also be noted that, in contrast to nanoparticles in solution, in the case of nanocomposites, an additional tool that increases sensitivity of the determination is increasing volume of the analyzed solution.

Morphology

Another factor in this system which should be considered is the morphology of AuNPs. To compare AuNPs of different shape, we consider here capabilities of spherical AuNPs and AuNRs. Since we aim to discuss on the aspect of morphology, these two kinds of AuNPs should be considered in processes leading to change in their shape. One of such processes is covering of nanoparticles by metal layers. As an example, we have considered an interaction of catecholamines (dopamine, noradrenaline, adrenaline, and dobutamine) with silver nitrate in the presence of AuNRs. As a result of this interaction, a hypsochromic shift of absorption bands of AuNRs occurs. Along with this, the color of the solutions changes significantly. It was proposed by us earlier to use this effect for spectrophotometric determination of catecholamines [14]. The mechanism of these changes involves reduction of silver ions by catecholamines and deposition of metallic silver on the surface of AuNRs forming Au@Ag core-shell structures.

Comparison of AuNRs and spherical AuNPs in this process (by the example of interaction with adrenaline under various conditions) allows one to conclude that spectral changes in the case of AuNRs are much more abundant than for spherical AuNPs (Fig. 4). A pronounced shift of absorption bands can be noted for AuNRs whereas only some increase of their intensity can be found in the case of spherical AuNPs. The color changes of solutions are also more remarkable for AuNRs. The consequence of all this is the better analytical characteristics of AuNRs compared to spherical AuNPs, which is especially clearly seen for nanocomposites based on PUF (Table 5).

Fig. 4:

Absorption spectra of AuNRs (a) and spherical AuNPs (b) before (1) and after interaction with 2 μM adrenaline solution at 25°C (2) and 70°C (3). The color changes are illustrated by color of corresponding lines.