Sythesis , Modification , and Biosensing Characteristics of Au 2 S / AuAgS-Coated Gold Nanorods

1 Institute of Photonics, Faculty of Science, Ningbo University, Ningbo, Zhejiang 315211, China 2School of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China 3Fujian Provincial Key Laboratory for Photonics Technology, Key Laboratory of Optoelectronic Science and Technology for Medicine of Ministry of Education, Fujian Normal University, Fuzhou 350007, China 4School of Information Science and Engineering, Ningbo University, Ningbo, Zhejiang 315211, China


Introduction
Noble metal nanoparticles (mainly Ag, Au, Pd, and Pt) have attracted more attentions due to their distinct physical and chemical properties, such as the quantum effect, the small size effect, large surface-to-volume ratio, and high surface chemical activity [1,2].Among them, gold nanoparticles have been widely used in biological and optical sensing [3], cancer imaging [4], chemical catalysis [5], and message storage [6,7], which is ascribed to their localized surface plasmon resonance (LSPR) and biocompatibility properties.In particular, over the past decade, many groups have focused their researches on the synthesis mechanism, surface functionality, and relative biological application of gold nanorods (GNRs) [8][9][10][11].It is because the LSPR properties of GNRs are easily tuned by simply changing the aspect ratio of the nanorods besides their high optical absorption cross sections and great molar extinction coefficients in ultraviolet-visible region.
Compared with gold nanospheres, GNRs are commonly characterized by two principal plasmon absorption bands, corresponding to the transverse plasmon resonance (TPR) and the longitudinal plasmon resonance (LPR) modes which are from the oscillation of conduction electrons [8,9].And the LPR band of GNRs is highly sensitive to the refractive index change of surrounding medium, which has been developed into an effective exploiter of biosensors [12][13][14].
On the other hand, many methods have been reported for preparing GNRs, for example, lithographic fabrication, template-assessed synthesis, and ultraviolet photochemical reduction of the gold salts [15][16][17].In fact, the most usual way to synthesize GNRs is the wet chemical seed-growth method due to its advantages of the facile operation and the mild aqueous reaction conditions [18,19].More importantly, by using the seed-mediated approach, the size and morphology of GNRs can be tailored by varying the ratio of seed to metal salt or coating other materials on the surface of the GNRs [8,9].As a result, the LSP band of GNRs can be tuned in a wide wavelength region [20,21].As we know, plasmonic sensors based on the LSPR characteristics of GNRs are highly sensitive to the changes of refractive index caused by molecular interactions in the vicinity of the GNRs [22].As reported [23], the absorption band of the GNRs with high-aspect-ratio is located in the "window of optical transparency," that is, in near-infrared region of 700-900 nm, which can achieve stronger optical absorbance and greater tissue penetration [23,24].However, it is cost-consuming and requires the strict synthesis conditions to obtain the GNRs with high-aspect-ratio [25,26].Therefore, it is crucial to adjust the surface plasmon resonance band of GNRs to nearinfrared region for their promising biological application.
In this paper, we provided a facile way to modulate the localized surface plasmon resonance band of gold nanorods into near-infrared region by coating a chalcogenide layer on the surface of gold nanorods, namely, Au 2 S/AuAgScoated gold nanorods (Au 2 S/AuAgS/GNRs).And the synthesis mechanism of the hybrid nanorods was discussed in detail.Then, to the functionalization of Au 2 S/AuAgS/GNRs, mercaptoundecanoic acid (MUA) and poly(styrenesulfonate) (PSS) separately modified the surface of CTAB-stabilized GNRs.Finally, the above functionalized Au 2 S/AuAgS/GNRs were incubated with anti-prostate-specific antigen (PSA) antibody to form the nanoprobe for detecting PSA in aqueous solution.As a consequence, the functionalized Au 2 S/AuAgS/GNRs showed an excellent performance as a label-free nanosensor platform for early-phase diagnosis of disease progression based on LSPR characteristics.Milli-Q water (18 MΩ cm resistivity) was used for all solution preparations.Glasswares were cleaned by aqua regia and rinsed with deionized water prior to the experiment.

Synthesis of GNRs.
The gold NRs were prepared by using a seed-mediated growth method [8,9] in aqueous solutions.Briefly, the seed solution was generated by adding ice-cold NaBH 4 (0.60 mL, 0.01 M) aqueous solution to the mixture solution of HAuCl 4 (0.25 mL, 0.01 M) and CTAB (7.5 mL, 0.1 M), followed by rapid inversion mixing for 2 min into a brown yellow solution.The growth solution, containing CTAB (47.5 mL, 0.1 M), HAuCl 4 (2 mL, 0.01 M), AgNO 3 (0.30 mL, 0.01 M), and AA (0.32 mL, 0.1 M), was prepared and then immediately changed to colorless after the AA was added.Finally, 0.1 mL seed solution was added to the growth solution and the reaction mixture was gently agitated for 20 s and left undisturbed overnight.[27,28], a modified strategy is used to coat a chalcogenide layer on the surface of GNRs.The Au 2 S/AuAgS/GNRs were prepared by adding Na 2 S 2 O 3 (0.055 mL, 1.0 M) solution into 5 mL of the as-synthesized gold nanorods at room temperature, heated, and kept at 50 ∘ C with different reaction time ranging from 20 s to 3 h.Then the mixture of GNRs and Na 2 S 2 O 3 was taken out, cooled to room temperature, followed by centrifugation at 10000 rpm for 20 min, and redispersed in 5 mL water.In our experiments, the as-prepared Au 2 S/AuAgS/GNRs were highly dispersed in water without aggregation during several weeks.

Modification of Au 2 S
/AuAgS/GNRs.The surface modification of Au 2 S/AuAgS/GNRs was carried out with MUA and PSS, respectively.In brief, MUA (0.6 mL, 0.01 M) ethanol solution was added into the 5 mL purified Au 2 S/AuAgS/ GNRs for reacting for 20 h.Then the modified nanorods were washed by centrifugation and resuspended in 5 mL water.The PSS monolayer was formed on the Au 2 S/AuAgS/GNRs by mixing PSS (0.5 mL, 0.01 M) solution with the 5 mL hybrid nanostructures and then rinsed by centrifugation at 10000 rpm for 20 min and resuspended in 5 mL water.

Immunoassay of PSA.
To attach biological molecules on the surface of the modified Au 2 S/AuAgS/GNRs, the EDC and NHS solution was firstly added to the modified nanorods and then incubated in an anti-PSA antibody solution (0.02 mg/mL) for 30 min.Then, a blocking solution BSA (0.02 mg/mL) was added to above prepared nanoprobes to shield the bare sites on the surfaces of the modified Au 2 S/AuAgS/GNRs, followed by centrifugation at 6000 rpm for 30 min and dispersion in PBS solution.The prepared nanoprobes were further used for detecting PSA.

Characterization and Measurement.
Transmission electron microscope (TEM) images were obtained with the TEM (JEM-2100F, JEOL) operating at an accelerating voltage of 200 kV.And UV-Vis absorption spectra were measured with the spectrometer (TU1901, Pgeneral).-potential measurements were performed with the nanoparticle size analyzer (Zetasizer Nano S90, Malvern).Powder X-ray diffraction (PXRD) data were collected on the Bruker D8 Focus X-ray diffractometer with nickel-filtered Cu K1 ( = 1.54060Å) radiation.The calculated PXRD patterns were produced using the SHELXTL-XPOW program.Energy-dispersive Xray (EDX) spectroscopy was carried out at 5 kV using a SU-70 field-emission scanning electron microscope (SU70, FESEM).

Result and Discussion
3.1.Characterization of GNRs and Au 2 S/AuAgS/GNRs.The GNRs were synthesized by the seed-mediated growth method with the help of surfactant-directed shape [8,9], and their TEM image is shown in Figure 1(a).It is seen that the assynthesized GNRs have an average aspect ratio of 3.1.The chalcogenide layer is coated on the surface of GNR by mixing an excess of freshly prepared Na 2 S 2 O 3 aqueous solution with the GNRs.Then the gold core-shell nanorods with chalcogenide complexes were obtained and shown in Figures 1(b)-1(d).Comparing their TEM images in Figure 1, it is clear that the average size of the gold nanorods coated with chalcogenide seems to be smaller than that of the original gold nanorods due to the corrosion of gold nanorods.Besides, it is seen from Figures 1(b)-1(d) that the morphologies of the core-shell nanocomposite are fairly uniform in shape and the average aspect ratio of the core-shell nanorods is increased with increasing the reaction times.Thus, it illustrates that the corrosion occurred both at the sides and the ends of gold nanorods and eventually led to the formation of high aspect ratio nanorods, which is different from the conventional formation of core/shell structure by coating the shell on surface of the nanorods.Moreover, the energy-dispersive X-ray (EDX) spectrum and the powder X-ray diffraction (PXRD) spectrum of the gold nanorods with chalcogenide complexes are shown in Figure 2. As shown in Figure 2(a), it clearly displays that the sample is mainly composed of the elements Au, Ag, and S, and the elements of silicon and bromine are from the silicon substrate and the surfactant CTAB.Meanwhile, the PXRD spectra shown in Figure 2(b) further identify the chalcogenide complexes of Au 2 S and AuAgS according to the previous reports [27].Compared with the PXRD spectrum of GNRs, it is found that the peaks of surfactant CTAB on the surface of GNRs disappeared and a weak peak of the Au 2 S/AuAgS complexes arose in the PXRD spectrum of Au 2 S/AuAgS/GNRs.Thus, it confirms that the Au 2 S/AuAgS/GNRs hybrid nanoparticles are successfully fabricated.
On the other hand, during the preparation process of Au 2 S/AuAgS/GNRs, their absorption spectra were recorded  with increasing the mixing time of the GNRs and Na 2 S 2 O 3 , as shown in Figure 3(a).As expected, the GNRs possess two principal plasmon absorption bands, a weak transverse plasmon resonance band at 520 nm, and a strong longitudinal plasmon resonance (LPR) band at 765 nm.After adding the excessive Na 2 S 2 O 3 to the GNRs solution, the LPR band red-shifts immediately from 765 nm to 792 nm within 5 minutes due to the fact that the chalcogenide layer was coated on surface of GNRs, while transverse plasmon resonance band red-shifts only several nanometers, as shown in the curves A-C of Figure 3(a).Furthermore, the curves (D-F) in Figure 3(a) also clearly show that the LPR bands have a bit of blue shifts with increasing the reaction time of the original GNRs.This is because there is a slight change of the aspect ratio of the core-shell nanorods during the first 13 minutes.Additionally, the more vigorous reaction of the GNRs and Na 2 S 2 O 3 may occur in the mixture solution when the reaction temperature reached 50 ∘ C after heating for 13 minutes, which results in the blue shifts of the LPR bands, which was shown in the curves G-J of Figure 3(a).In particular, for the case of curves K in Figure 3(a), the LPR band suffered a significant blue shift as well as a dramatically decline of the spectrum intensity.The distinct change of LPR bands is ascribed to the continuous corrosion of Na 2 S 2 O 3 which makes the average length and diameter of the GNRs naturally decreased, further resulting in the decrease of the aspect ratio of the GNRs.Therefore, the above shift of LPR bands is different from the consecutive red shifts of the LPR bands with increasing the reaction time which was described in [27].
It is well known that the as-synthesized GNRs are encapsulated with CTAB which shapes and stabilizes the nanorods [29].In fact, the excess CTAB in the GNRs solution is detrimental to the synthesis of core-shell nanorods such as the Au/Ag and Au/SiO 2 nanorods [20,30].However, as shown in Figure 3(b), when the purified GNRs reacted with the Na 2 S 2 O 3 , the LPR band of Au 2 S/AuAgS/GNRs had only blue shifts no matter for how long was the reaction carried out.Compared with the case of GNRs without centrifugation, the LPR bands in Figure 3(b) have no red shift during the process of mixing the GNRs and Na 2 S 2 O 3 .It is indicated that the CTAB plays an important role in tuning the LPR band to near-infrared and slowing down the dramatic blue shifts because the CTAB as a blocking agent stopped the Na 2 S 2 O 3 molecular directly contacting with the surface of GNRs.

Functionalization of Au 2 S/AuAgS/GNRs with PSS and MUA.
In general, during the preparation of GNRs and Au 2 S/ AuAgS/GNRs, the CTAB as a surfactant is coated on the surface of the nanorods.However, the CTAB has significant cytotoxicity to the biological tissue and blocks the nanorods to bind biological molecules.So the functionalized modification of the nanorods is important for their applications.In our research, MUA and PSS molecules are chosen as the modifier to functionalize the Au 2 S/AuAgS/GNRs.The scheme of MUA and PSS functionalized with Au 2 S/AuAgS/GNRs is illustrated in Figure 4.As described in Section 2.4, the MUAcapped and PSS-capped Au 2 S/AuAgS/GNRs are, respectively, centrifuged and redispersed in water to remove free CTAB and the excessive MUA/PSS.Then, for the functionalized Au 2 S/AuAgS/GNRs, the displacement of CTAB to MUA/PSS was monitored by -potential measurement.As shown in Table 1, the GNRs and Au 2 S/AuAgS/GNRs coated with CTAB are positively charged during the preparation of GNRs.However, after coating MUA layer on the surface of Au 2 S/AuAgS/GNRs, the zeta-potential increases 6.6 mV more than that of Au 2 S/AuAgS/GNRs due to the thiol group attached to the nanorods and the formation of layers of alkanethiol molecules [31,32].Meanwhile, the zeta-potential of PSS suffers an extreme change from +30.6 mV to −44.3 mV, which was from the interaction between the positive charge of CTAB and the negative charge of PSS [33,34].It is because the electrostatic interaction between the anionic SO 3 groups of PSS and the trimethylammonium headgroup of CTAB results in the extreme change in the -potential which switched from positive to negative [35].
The optical properties of the functionalized layers coated on the surface of Au 2 S/AuAgS/GNRs were also characterized by UV-vis-NIR absorption spectroscopy.As shown in Figure 5, the LSP bands of Au 2 S/AuAgS/GNRs and the functionalized Au 2 S/AuAgS/GNRs are located in 802, 807, and 798 nm, respectively.Comparing with Au 2 S/AuAgS/GNRs, the LSP band of MUA-capped Au 2 S/AuAgS/GNRs has 5 nm red shift but the one of PSS-capped Au 2 S/AuAgS/GNRs has 4 nm blue shift.The above shifts can be attributed to the changes of refractive index caused by replacing CTAB with MUA/PSS molecules in the vicinity of the Au 2 S/AuAgS/GNRs [31,32], as shown in the variations of -potentials of the functionalized Au 2 S/AuAgS/GNRs in Table 1.It further confirms that the functionalization of Au 2 S/AuAgS/GNRs with PSS and MUA has been achieved, which not only diminishes the cytotoxicity of CTAB but also makes Au 2 S/AuAgS/GNRs easy to attach with biomarker molecules as biosensing probes for the biological detection later.

Application of Au 2 S/AuAgS/GNRs Immune Probes.
As an immunoassay application, the prepared MUA-capped Au 2 S/AuAgS/GNRs were conjugated with the anti-PSA antibody to form the immune probe for specific binding with PSA to monitor early prostate cancer [36].Before attaching anti-PSA antibody, the EDC and NHS biomolecules as the coupling agents were added to the MUAcapped Au 2 S/AuAgS/GNRs solution so that the EDC/NHSterminated nanorods are easy to covalently attach with the anti-PSA antibody.As shown in Figure 6, the absorption spectrum of MUA-capped Au 2 S/AuAgS/GNRs conjugated with anti-PSA antibody showed an obvious decrease of intensity and a slight red shift of the LSP band.After adding 10 L of 0.04 mg/mL PSA to 5 mL of the immune probe solution, the LSP band of the immune probe incubated with PSA was further declined and broadened to a longer wavelength.It is believed that the above results come from the specific binding of PSA with the anti-PSA antibody of the immune probes.
In addition, the PSS-capped Au 2 S/AuAgS/GNRs were also employed to perform the immunoassay with the above protocol.However, after adding the anti-PSA antibody into the PSS-capped Au 2 S/AuAgS/GNRs solution, there was no shift of the LSP band of the nanorods solution.It indicated that the anti-PSA antibody might not be attached on the PSS-capped nanorods.In other words, the PSS-capped Au 2 S/AuAgS/GNRs do not act as the immune probes to detect PSA.But the PSS-coated Au 2 S/AuAgS/GNRs may be useful to detect other biomolecules by binding with the corresponding target analytes.Therefore, the different biorecognition driven by the MUA-capped and PSS-capped Au 2 S/AuAgS/GNRs immune probes reveals a good selectivity of biomolecules by functional Au 2 S/AuAgS/GNRs, which may be an important capability for multiplex biosensor.

Conclusion
In we have successfully synthesized Au 2 S/AuAgScoated gold nanorods in a simple and efficient way.And CTAB plays an important role in controlling the corrosion rate of GNRs, leading to different changes of LSP band of the nanocomposites.With excessive CTAB molecular in GNRs solution, the LSP peak of the as-prepared Au 2 S/AuAgS GNRs firstly red-shifts and then blue-shifts as the reaction time between Na 2 S 2 O 3 and GNRs increases.After removig the excessive CTAB by centrifugation, the purified GNRs were mixed with the Na 2 S 2 O 3 solution; the LSP peak only tends to blue-shift.Moreover, the MUA-and PSS-coated Au 2 S/AuAgS GNRs exhibit different chemical reaction mechanisms, which are achieved by thiol group bonding and electrostatic interaction, respectively.Experimentally, it demonstrated that the MUA-coated Au 2 S/AuAgS GNRs can be prepared as immune nanoprobes conjugated with anti-PSA antibody to detect PSA because its LSP band has red shift and broadening accompanied with an intensity decrease.In principle, the Au 2 S/AuAgS GNRs as an effective nanoprobe may be extended to assay other biomarkers based on their LSPR properties in NIR region.
Au 2 S/AuAgS/GNRs.Based on the research work of Huang et al. presented in