Abstract

We determine the surface plasmon resonance (SPR) band values of nanorods gold nanoparticles as function of silver nitrate and seeds solution volume.Seed-mediated growth process is used to prepare gold nanorods. The influence of silver nitrate and seeds solution volume was studied using the characterizations techniques: UV-Vis-NIR and TEM. The results showed the nanoparticles obtained increasing silver nitrate and seeds solution volume were proved to produce a maximal longitudinal SPR (SPRL) response and good dispersion. Homogeneous nanorods were obtained under conditions with optimized seed volume.The SPR performance of the nanoparticles was systematically evaluated using volume silver nitrate (80 l to 160l) and volume seeds (15 l to 45l).The resonance frequency show an evolution toward high wavelength, results for these structures demonstrated potential applications in windows optical.

1. Introduction

Nanoparticles are considered to be the most valuable and important functional materials in the field of material science. Noble metal nanoparticles (Au, Ag and Cu) have been widely studied in the past decades due to their potential applications in many areas. The areas of application of the noble metal nanoparticles are extremely diverse (catalysis ¹, optics ^{2, 3}, nanomedicine ³, biology ³. A nanoparticle is defined as a tiny particle with a size ranging between 1 and 100 nm.They also exist in different shapes such as nanorods, nanospheres, nanostars, nanocluster, nanocube, nanoprisms, tetrahedral, nanotriangles, etc. ^{4, 5}. The color of gold nanoparticles mainly depends on its size. If size of particle is large it appears more red in color ⁶.

Gold nanorods have been established as central items for biological applications over several decades. Their ability to absorb light upon irradiation originates from the collective oscillation of electrons in the conduction band of the gold surface; it results in two localized surface plasmon resonance bands called the transversal and longitudinal bands. For gold nanorods, transversal resonance is located in the green spectral region at around and the longitudinal shifted to the infrared region (650 nm to 1100 nm) and strongly depends on the aspect ratio of nanorods. As aspect ratio is increased, the longitudinal peak is redshifted ^{7, 8}.

Transversal mode, weaker band localization is similar to the gold nanosphere( peak observed aroud 520 nm). Moreover, the position of longitudinal band displays a higher sensitivity to the variation of the dielectric environment ⁷, composition (core/shell) ⁸, size ^{8, 9}, chemical product concentrations ⁹. Ming-Zhang Wei and co-workers ⁹, show that the longitudinal band has increasing in a red shift from 615 to 869 nm at the very low cetyltrimethylammonium bromide (CTAB) concentrations (0.008 to 0.010 M). This study highlights an important consideration regarding the experimental characteristics of the chemical compounds to be used for optical studies of gold nanparticles.

Several synthesis methods of gold nanorods have been developed, including chemical reduction ¹⁰, electrochemical reduction ¹¹ and photochemical reduction ¹². They are typically synthesized via reduction of gold salts such as hydrogen tetrachloroaurate trihydrate (HAuCl₄) using trisodium citrate reducing agents in aqueous solution for Turkevich method, this method is one of the most prominent approaches for synthesizing spherical gold nanoparticles and using hexadecyltrimethylammonium bromide (CTAB), reducing agent used for synthesis gold nanorods nanoparticles by seeded growth method. Nanospheres are the simplest ans most symmetric gold nanoparticles, characterized by their spherical geometry and uniform surface.

The Turkevich method is the first chemical method of gold nanoparticles synthesis described by Turkevich in 1951. It is on of the most well known techniques used for the synthesis of spherical AuNPs within a range of 10-30 nm ^{13, 14} and seeded growth method, using a strong reducing agent such as sodium borohydride, NaBH₄ to from seed particles and then these seed particles were added to the metal solution containing weak reducing agent (ascorbic acid). This study highlights an important consideration regarding the experimental characteristics.

In this study, we conducted a comparative analysis of the possible mode on localized surface plasmon resonance in nanorods gold nanoparticles. Systematic control of band plasmonic through a seed mediated growth method by varying the silver nitrate, AgNO₃ volume and seed volume, allowing pricise optimization of plasmonic properties. The resulting nanorods nanoparticles were characterized using TEM and UV-vis spectroscopy. The paper is structured as follows. In section 2, we present experimental section, as materials and synthesis of materials. In section 3, we report optical properties results obtained by experimental. Morphology and chemical composition are key parameters that conditioned the final optical properties and control plasmonic band.

2. Experimental

Tetrachloroauric acid (HAuCl₄. 3H₂O), sodium borohydride (NaBH₄, 99%), cetyltrimethyl ammonium bromide (CTAB, [(CH₃(CH₂)₁₅N(CH₃)₃]B, 99%), sodium citrate dibasic trihydrate (Na₃C₆H₅O₇), silver nitrate (AgNO₃, 99%), hydrochloric acid (HCl, 37%), ascorbic acid (A A, C₆H₈O₆, ≥ 99%), Milli-Q grade water were used in all preparations as solvent. All glassware used in the experiments was cleaned, washed thoroughly with Milli-Q water,and dried before use. Tetrachloroauric acid is also stored in the refrigerator, protected from sunlight, for a long period of time. However, AgNO₃ and ascorbic acid solutions should be discarded after each experience.

Small size gold nanoparticles were prepared by the well-known Turkevich method. 2.5mL of Chloroauric acid (HAuCl₄. 3H₂O 0.01M) was mixed with 47.5mL of Milli-Q water. The mixture was heated till boiling began at 90°C (under reflux) and in that moment 25mg of sodium citrate dibasic trihydrate (Na₃C₆H₅O₇) dissolved in 2.5mL of Milli-Q water was added all at once under vigorons stirring. Then, the system was heated up to 130°C at a constant rate, and the reaction allowed to proceed for 1h at this temperature and then the reaction mixture was cooled to room temperature. Sodium citrate acts as both reducing and stabilizing agent. It is a charged molecule that attaches itself to the surface of nanoparticles and ensures electrostatic repulsion. The color of the suspension changed from light yellow to grey and then to red, resulting the formation of spherical gold nanoparticles, stabilised by the electrostatic interactions between the gold surface and the citrate ions.

It can be done by two general colloidal approaches such as seed-mediated method and seedless growth ^{9, 15, 16}.

Gold seed solution was prepared by borohydride reduction. At room temperature (ca. 25°C), 50µl HAuCl₄ 0.025 mM is added to 4.7ml CTAB 100mM. Since CTAB crystallises at room temperature, it must be solubilised by stirring in a thermostatic bath at 30°C, ensuring that the bottle containing the solution is completely immersed in the bath to prevent foaming. The CTAB solution is kept in the bath throughout its use.The mixture is stirred magnetically for 5 minutes.

After this, 300µl of a freshly prepared NaBH₄ 0.01M (19mg in 50ml of Milli-Q water) solution is rapidly injected in the previous solution. The solution was vigorouly stirred for 1 min to produce a light brown solution, which serves as the seed solution. Sodium borohydride role is as a reducing agent that reduces Au³⁺ to Au. Finally, the flask solution is left at 30°C in water bath for 2 hours for the complete decomposition of NaBH₄.

Firstly, 10 ml of 0.01M silver nitrate solution (17mg of AgNO₃ in 10ml H₂O) and 10 ml of 0.1M ascorbic acid (176mg of C₆H₈O₆ in 10ml H₂O) were prepared. 200µl HAuCl₄ 0.025M is added to 4.625ml of Milli-Q water. In order, we have injected 5ml CTAB 0.2M, 80µl AgNO₃ 0.01M, 15.4µl HCl 37% and 80µl AA 0.1M, respectively. After the addition of ascorbic acid (AA), the solution is gently shaken by hand, the mixture turns colorless in few seconds. Reduction of Au³⁺ to Au¹⁺ results in disappearance of color. Then, 24µl of the seed solution is added, the solution is vigorously shaken by hand and left undisturbed for 2 hours at 30 °C in water bath.

Morphology of nanorods was evaluated by transmission electron microscopy (TEM) and the ultraviolet visible near infrared (UV-Vis-NIR) for to measured surface plasmonic resonance and check the repeatabily of synthesis, aliquots of colloïdal solution.

Transmission Electron Microscopy was carried out with a JEOL JEM-2100 PLUS. Transmission electron microscope operating at an acceleration voltage of 100 kv.

The UV-Vis-infrared spectra were recorded on a double beam Cary 5000 spectrophotometer in steps of 1nm in the 300-800 nm using 1 cm quartz optical cell. In the technique, gold nanoparticles synthesis were envoloped with aluminium foil and cooled for about 30 minutes. Takes 2,5 ml of solution, dilue it slightly with Milli-Q water, and place it in the ultra-violet visible spectroscopy for characterization after calibration with the control sample. Spectra were mathematically fitted with Origin software.

3. Results and Discussion

Experimentally, optical properties were measured in the wavelengths region from visible (300-800 nm). Gold nanoparticles have unique optical properties due to the surface plasmon resonance (SPR) phenomenon. SPR is a resonant phenomenon of oscillation or vibration of metal valence electron on the surface of nanoparticles ¹⁷.

In this section, we have associated turkevich protocol to two different mass of sodium citrate dibasic trihydrate (Na₃C₆H₅O₇). Optical extinction spectra of gold nanosphere (AuNPs) colloidal solutions were measured for 25 mg of Na₃C₆H₅O₇, and for 27 mg of Na₃C₆H₅O₇, for figure 1.A and figure 1.B, respectively. Citrate concentration dit not significantly affect the width and the intensity of the resonance band as shown in figure 1.Gold nanoparticles (red solution), the peaks are located at about 520 nm and at 522 nm, for 25 mg of Na₃C₆H₅O₇ and 27 mg for Na₄C₆H₅O₇, respectively, ¹⁸. Indicative of spherical and well dispersed AuNPS. The blue shift observed in the spectra suggests a smaller particle size ¹⁸.

Moreover, such results are in accordance with the works of Mulvaney et al ¹⁹ and Ringe et al ²⁰. The absence of secondary bands confirms the absence of aggregates and the good dispersion of the nanospheres in both samples. These results indicate that the main distinction between the spectra lies in the position of the plasmonic peak, reflecting the difference in average size and homogeneity of the nanospheres induced by the amount of citrate. Spectral shifts and peak broadening reveal varying degrees long term stability among these mothods, offering insight experimental sensitivity to surface plasmonic effects. It is well know through that the plasmon resonance, is very sensitive to change the citrat quantity. The band shift to the longer wavelengths with increasing citrate mass.

Optical characterization of gold nanorods (AuNRs) was carried out by UV/vis spectroscopy, figure 4, TEM image, figure 5 and experimental quantitative data during the AuNR formation Table 1. Gold nanorods Exhibit two peaks, one at ~520 nm and the other at ~700-1000 nm, depending of AgNO₃ 0.01M volume. These peaks correspond to transverse and longitudinal plasmon resonance modes: the shorter-wavelength peak (around 520 nm) corresponds to electron oscillations in the transverse direction, while the longer-wavelength peak (700-1000 nm) corresponds to longitudinal oscillation.

To fully control the synthesis of the AuNRs, we further investigated the influence of AgNO₃ on AuNRs synthesis. First, we fixed the other parameter et only changed the volume of AgNO₃ (Table 1).

Three samples named A1, A2 and A3 were synthesised by varying only the amount of silver nitrate from 80, 120 and 160, respectively. UV-vis and TEM analysis were used to evaluate the effect of this variation on the formation of nanorods. The process and components for each samples are listed in the Table 1.

In this study, the maximum wavelength and surface plasmon resonance of longitudinal SPR_L band of the three samples were found in the range of 600 – 1000 nm. The maximum wavelength and SPR_L are strongly influenced by the volume of AgNO₃.It can be clearly seen from figure 4, that high SPR_L in the infrered range is obtained. The SPR_L band wavelengths are at 690, 770 and 860 nm (figure.4) for 80, 120and 160 of AgNO₃ for sample A1, Sample A2 and Sample A3, respectively. These results indicate that the plasmon band increase with volume AgNO₃.

TEM images (figure 5) confirm these spectral observation: sample A1 showsa large population of well defined nanorods with very few spherical particles, sample A2 reveals generally homogeneous rods with a few aggregates and a slightly more spread out length distribution with the presence of nanospheres, while sample A3 shows a high density of regular, well formed nanorods without major aggregates, reflecting the most efficient anisotropic growth in the series.

To explore the plasmon response of the AuNRs, we performed a series of experiments using differents volumes of the seed solution at a fixed CTAB concentration (0.2M). The experimental parameters are listed in Table 2, the extinction spectra are shown in figure 6 and TEM images of AuNRs are shown in figure 7.

To fully control the synthesis of the AuNRs, we further investigated the influence of seed on AuNRs synthesis. The volume of seed is 15, 30 and 45 for the Sample A4, Sample A5 and Sample A6, respectively. This seed-mediated growth approach is a significant wet chemical method to achieve AuNRs with controlled size and shape, which is the state of the art method for the synthesis of AuNRs ²¹. The corresponding wavelengths for the three sample are 865 nm, 870 nm and 895 nm, indicating a strong red shift of the plasmon response with the increase of the quantity of seeds added. There is still a wide range of tunability (from 865 to 895 nm) at the seed volumes (15-45). The position of the SPR_L peaks at 865 nm (Sample A4), at 870 nm (Sample A5) and at 895 nm (Sample A6).

As can be seen from the TEM images in figure 7 (Sample A4), when the amount of seeds added to the growth solution was 15µl, Au nanorods containing some irregularly shaped coproducts were obtained. When the amount of the seeds was increased to 30µl, then 45µl the content of the coproducts was decreased (Sample A5 and Sample A6), the gold prepared were homogeneous in shape and size, with good monodispersity. We can conclude that the optical extinction spectrum shifts toward near infrared region with the increasing seeds volume. Theses results are according to Ming-Zhang Wei and al. ⁹ work. They demonstrated that on increasing seeds volume, the surface plasmon resonance re-shifted.

It is well known through that the plasmon resonance is very sensitive to experimental parameters as AgNO₃ and the seeds. The spectra information deduced from the UV-Vis infrared analysis shows according to the results reported by A Sambou and co-authors ²², the plasmon band shifts toward the higher wavelength region when nanorods size increases. From the results obtained, we can say that the increase in the volume of AgNO₃ and seeds induce an increase in nanorod size. This study proved that the addition of AgNO₃ and seed was favorable for the formation of gold nanorods much long.

4. Conclusion

We have experimentally studied a optical properties based on localized surface plasmon resonance in nanorod Au nanoparticle by using a seed-mediated growth method with the addition of various quantity of silver nitrate and seeds. The shape and position of the nanostructure SPR_L extinction peak are highly dependent on expermentals data. As the volume of AgNO₃ and seed increase, the plasmonic band of the nanorods redshifts and the nanorod size increases. It can be concluded that the chemical components an important factor on the formation of gold nanorods. The nanoparticles were synthesized by a seed mediated growth method and exhibited size and chemical composition dependend plasmonic properties. TEM imaging confirmed the formation of gold nanorods structures and UV-Vis spectroscopy revealed shifts of localized surface plasmon resonances.

ACKNOWLEDGMENTS

This work was suported by Cheikh Anta Diop University of Dakar (Senegal) and we would like to thank the laboratoir «Interfaces Traitements Organisation et DYnamique des Systèmes (ITODYS)» for Paris City University for experimental work.

References