Size Control Synthesis of Monodisperse, Quasi-Spherical Silver

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Functional Nanostructured Materials (including low-D carbon)

Size Control Synthesis of Monodisperse, Quasi-spherical Silver Nanoparticles to Realize SERS Uniformity and Reproducibility Lixiang Xing, Yujiao Xiahou, Peina Zhang, Wei Du, and Haibing Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02052 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Materials & Interfaces

Size Control Synthesis of Monodisperse, Quasispherical Silver Nanoparticles to Realize SERS Uniformity and Reproducibility Lixiang Xing,a Yujiao Xiahou,a Peina Zhang,a Wei Du,b and Haibing Xia*,a a

State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, P. R. China.

E-mail: [email protected] b

School of Environment and Material Engineering, Yantai University, Yantai 264005, Shandong,

China. KEYWORDS: silver nanoparticles, size control, SERS, uniformity, reproducibility

ACS Paragon Plus Environment

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ABSTRACT: In this work, we reported the synthesis of monodisperse, quasi-spherical Ag nanoparticles (NPs) with sizes of 40 to 300 nm by using ascorbic acid (AA) reduction of silverammonia complex onto pre-formed, 23 nm Ag-NP seeds in the aqueous solution with an optimal pH of about 9.6 at room temperature. The as-prepared Ag NPs with such a large size span (from 40 to 300 nm) and high quality by one-pot seeded growth method are reported for the first time, to the best of our knowledge. It is found that the key in the present seed-mediated growth method is to introduce a proper amount of ammonia water for formation of a stable complex with silver precursor (silver-ammonia complex) while maintaining pH value of the growth solution simultaneously. By using Rhodamine 6G (R6G) molecules as probes, the surface-enhanced Raman scattering (SERS) activities of as-prepared Ag NPs in ethanol solution are highly dependent on the sizes of Ag NPs at the fixed 633 nm laser and at the fixed particle number, which show a volcanolike curve. Moreover, 125 nm Ag NPs bear the largest SERS activity among them. Furthermore, Ag NPs with narrow distributions in shape and size (say, less than 10%) can achieve the uniformity and reproducibility of their SERS signals in solution; their relative standard deviations (RSDs) can be as low as 5% in space and temporal scale.


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1.

Introduction

Silver nanoparticles (Ag NPs) have attracted more research interests due to their unique physicochemical properties, which can be used in a series of potential applications, such as optoelectronics,1,2 catalysis,3,4 sensors,5,6 antibacterial medicines,7 surface-enhanced Raman scattering (SERS) detection.8,9 In addition, it has been demonstrated that the performance of the resulting Ag NPs in some applications is largely dependent on their size, especially in SERS.10–12 Currently, wet-chemical methods are usually used for the preparation of Ag NPs with different sizes.13–18 For instance, Yang and co-workers reported17 that the synthesis of Ag NPs with sizes of 7 to 66 nm by directly adjusting the ratio of tannic acid (TA) to silver nitrate and the pH of the system. Moreover, Ag NPs in the range of 79 - 200 nm were further produced by using the resulting Ag NPs as seeds via multi-step growth method. However, multi-step injection growth method is still troublesome and complicated somehow.19 Thus, one-pot seeded growth method was developed to prepare large-size Ag NPs. For example, Steinigeweg and co-workers reported20 that Ag NPs with sizes of 40 to 100 nm were obtained by the reduction of silver-ammonia complex by ascorbic acid (AA) in the presence of polyvinylpyrrolidone (PVP) onto Ag-NP seeds of about 30 nm, which were firstly prepared by reduction of silver ions by sodium citrate in the presence of glycerol. Gao and co-workers also synthesized Ag NPs with diameters upto 140 nm by using 3 nm Au NPs as seeds in the presence of PVP and acetonitrile as protective agents.21 Although the onepot seeded growth has achieved the preparation of large-size Ag NPs, the presence of organic ligands with strong affinity on the surfaces of the resulting Ag NPs would limit their applications to a certain extent.22 Moreover, the shape and size distribution of as-prepared Ag NPs has to be further improved to achieve the quantitative analysis and reproducibility in sensor and SERS


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applications.23–25 Therefore, it would be desirable to prepare Ag NPs with a large size span and coated by easily replaceable ligands in water through one-pot seeded growth method. According to the LaMer model,26–30 controlling the growth rate in the seeded growth method is the key to achieve the fabrication of uniform large-sized NPs, which should avoid the occurrence of self-nucleation after the addition of seeds under high concentration of precursors. For instance, Yin and co-workers achieved continuous seeded growth without self-nucleation by stabilizing precursors with strong coordinating ligands as the newly-formed complex can significantly decrease the reduction potential of original precursors.19,31,32 In previous works, silver nitrate is widely used as the precursor for the preparation of Ag NPs. However, due to the high chemical potential of silver nitrate, it is rather difficult to precisely control the growth rate of Ag NPs in the seeded growth method by selecting a suitable reducing agent to reduce, even hydroquinone (HQ) with a very weak reducing ability.33,34 Thus, it is may be a good option to control the growth rate of Ag NPs by judiciously selecting one silver precursor with lower chemical potential,35–38 instead of the reducing agent. Thus, in this work, we developed one new but facile and robust method to synthesize uniform, quasi-spherical Ag NPs with a large size span from 40 to 300 nm by direct AA reduction of silverammonia complex onto prep-formed, citrated-stabilized, quasi-spherical Ag-NP seeds of about 23 nm by our previous method39,40 in the aqueous solution with an optimal pH value of about 9.6. The effects of residual citrate ions in the dispersion of Ag-NP seeds, the concentrations of AA and ammonia water on the growth of Ag NPs were investigated. In the present seeded growth method, ammonia water can form complexes with silver precursor and buffer the pH of the reaction media simultaneous. The synergy of the two roles of ammonia water allows us to govern the growth rate of Ag-NP seeds in the one-pot seeded growth method, thus achieving size control of the final Ag


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NPs by simply adjusting the particle number of the Ag-NP seeds. To the best of our knowledge, this is the first work that can achieve the synthesis of quasi-spherical Ag NPs with such a large size span in water by one-pot seed-mediated growth method without using additional strong stabilizers. In addition, by using Rhodamine 6G (R6G) molecules as probes, the SERS activities and the uniformity and reproducibility in both in space and temporal scale of as-prepared Ag NPs in ethanol solution were evaluated at fixed excitation wavelength of 633 nm and at the fixed particle number. 2.

EXPERIMENTAL SECTION

2.1 Materials. Silver nitrate (AgNO3, 99.995%) was ordered from Alfa Aesar (Shanghai). Rhodamine 6G (C28H30N2O3·HCl, 95%) was purchased from Sigma-Aldrich. Sodium chloride (NaCl, 99.5%), trisodium citrate dihydrate (Na3C6H5O7∙2H2O, 99%), ascorbic acid (AA, 99.7%), sodium hydroxide (NaOH, 96%), ammonia water (NH3∙H2O, 25% ～ 28%), crystal violet (C25H30N3Cl) and ethanol absolute (C2H6O, 99.7%) were bought from Sinopharm Chemical Reagent Co. Ltd. All chemical reagents were used without further treatment. All glassware and stirring bars were cleaned by the same procedure, which was described in detail in our previous work.39,40 The Milli-Q water was used in all experiments. 2.2 Preparation of Citrate-stabilized 23 nm Ag NPs as Seeds (Ag-NP Seeds). Citrate-stabilized, 23 nm Ag-NP seeds with quasi-spherical shape were prepared according to the protocol reported in our previous work.40 Typically, 1 mL of the aqueous solution of sodium citrate (1 wt%), 0.25 mL of the aqueous AgNO3 solution (1 wt%), and 0.2 mL of the aqueous NaCl solution (20 mM) were successively added to 1.05 mL of water under stirring at room temperature. After five-minute premixing, the citrate-silver-NaCl premixture was quickly added into 47.5 mL of the boiling water. Note that 80 µL of the aqueous AA solution (0.1 M) has to be added into the boiling water one


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minute before the addition of the citrate-silver-NaCl premixture. After heating and stirring for 1 hour, the resulting solution was allowed to cool to room temperature, and a dispersion containing Ag NPs with a bright yellow color was finally obtained. The average size of the Ag-NP seeds was obtained by TEM result (Figure S1, Supporting Information) and their particle number concentration in the growth solution was calculated to be about 4.8×1011 mL-1, on the basis of the calculation method reported in our previous work.30 2.3 Controlled Synthesis of Ag NPs with Sizes from 40 to 300 nm. The solution of silverammonia complex was firstly prepared for synthesis of Ag NPs. Typically, 2 mL of the aqueous AgNO3 solution (1 wt%) was mixed with 800 µL of ammonia water (25%～28%). For the synthesis of 85 nm Ag NPs, typically, 200 µL of the original solution of Ag-NP seeds was added into water (4.73 mL) under stirring in a 10 mL glass vial at room temperature. Subsequently, the aqueous solution of silver-ammonia complex (70 µL, 43 mM) and the aqueous AA solution (2 mL, 2.5 mM) were added into 10 mL glass vial in succession. After stirring for 1 hour, the resulting Ag NPs were concentrated by the centrifugation and redispersed in the aqueous solution of sodium citrate (0.02 wt%) for storage. The sizes of other Ag NPs can be adjusted by varying the amount of Ag-NP seeds (particle number) used in the growth solution when the amount of silver-ammonia complex was fixed. All of recipes were shown in Table S1. 2.4 Preparation of Samples for SERS. Since R6G molecules are easy to form dimers and aggregate in water, Ag NPs of various sizes prepared by our method were dispersed into ethanol for SERS tests41 by the post-treatment of centrifugation and redispersion. Typically42, the samples for SERS were obtained by mixing of ethanol solution containing Ag NPs with different sizes (0.02 nM, 0.5 mL) and ethanol solution containing R6G probes (0.2 mM, 0.5 mL). Accordingly, the final concentrations of Ag NPs and R6G were 0.01 nM and 0.1 mM, respectively. The mixed


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solutions were then sucked into capillary glass tubes (inner diameter of tubes is about 1 mm) for SERS tests. 2.5 Theoretical Simulation of Extinction Spectra of Differently-sized Ag NPs. The extinction spectra of differently-sized Ag NPs of in water were simulated by Mie Plot software according to Mie theory. In theoretical simulation, the mean sizes and standard deviation rates of spherical Ag NPs were set appropriately according to the quality of the resulting Ag NPs actually obtained. The databases on the refractive indices of silver and water at different wavelengths originated from the works of Palik and Segelstein, respectively.43,44 2.6 Characterizations. The morphology and average sizes of Ag NPs were characterized by JEOL transmission electron microscope (TEM, JEM-2100F), which was operated at an accelerating voltage of 200 kV. The ellipticity and deviation in size of all samples were obtained by statistically counting more than 100 particles based on their corresponding TEM results. Their ellipticities are calculated based on the ratio of the major axis to the minor axis of the NPs. The extinction spectra of the resulting Ag NPs were measured by a Cary 50 spectrophotometer. Dynamic light scattering (DLS) was performed on a Malvern Zetasizer Nano ZS to get the size distributions of the resulting Ag NPs. The SERS measurements of R6G and CV were conducted by a Renishaw inVia Reflex Raman spectrometer with 633 nm laser at room temperature and their SERS spectra were recorded accordingly. 3.

RESULTS AND DISCUSSION 3.1 Variation of Redox Potentials of Silver Ions and AA in Ammonia Water. As mentioned

in literature,35,36 it is well known that the formation of silver-ammonia complex can greatly reduce the redox potential of silver ions. In addition, the redox potential of AA is also pH-dependent.45,46 Thus, the redox potentials of silver ions and AA in ammonia water will be altered. The evolution


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curves of the redox potentials of AA (C6H8O6), silver ions (Ag+) and silver-ammonia complex ([Ag(NH3)2]+) are drawn on the basis of reported data, as shown in Figure 1.35,45 After the formation of silver-ammonia complex ([Ag(NH3)2]+), the redox potential of silver precursors is reduced from about 0.79 V (pure Ag+ ) to 0.38 V ([Ag(NH3)2]+), at which the pH of reaction media is over about 8. Accordingly, the redox potential of AA is reduced from about 0.1 V (pH  5) to 0.002 V (pH  9) with pH increasing. Thus, in the current reaction solution containing ammonia water (pH  9.6), the difference of redox potential between [Ag(NH3)2]+ and AA is about 0.4 V, which is far lower that between pure Ag+ and AA (0.7 V at pH 4.5). The reduction in the difference of the redox potentials between the reactants can benefit to decrease the growth rate and then to regulate the growth of Ag-NP seeds in the seeded growth method. By AA reduction of [Ag(NH3)2]+ ions in the absence of Ag-NP seeds, the size distribution of Ag NPs obtained is rather broad; their average size of those big ones is up to 500 nm while that of those small ones is only 200 nm (Figure S2a). The result indicates that the growth rate of Ag NPs by AA reduction of [Ag(NH3)2]+ ions is rather slow, which results in the production of fewer nuclei at the initial stage and the big size of the final Ag NPs. In addition, the presence of Ag NPs of small size indicates that the secondary nucleation is unavoidable and the slow growth rate during the formation of Ag NPs. Accordingly, uniform Ag NPs can be obtained by depositing newly-forming Ag onto the small Ag-NP seeds (Figure S2b).


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Figure 1. Evolution of the redox potentials of AA, silver ions (Ag+) and silver-ammonia complex ([Ag(NH3)2]+) with pH values of the reaction solution. 3.2 Effect of Residual Citrate Ions in the Dispersion of Ag-NP Seeds on Synthesis of Ag NPs. The addition of Ag-NP seeds is essential for synthesis of differently-sized Ag NPs. However, the dispersion of Ag-NP seeds contains citrate ions, which also can react with silver ions at room temperature. Thus, it also may react with the sliver-ammonia complex, which would affect the quality of the resulting Ag NPs in the seeded growth method. Thus, the minimum content of the residual citrate ions in the dispersion of Ag-NP seeds was firstly investigated. In this patch of experiments, the particle number in the final dispersions of Ag-NP seeds is the same. However, the concentrations of the residual citrate ions in the final dispersion of Ag-NP seeds are different, which were obtained by concentrating the same volume of original dispersions of Ag-NP seeds


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into different volumes of the final dispersions of Ag-NP seeds. As expected, the ellipticity of the resulting Ag NPs by the seeded growth method is obviously impacted (Figure S3) although their average sizes are nearly the same (40 ±3 nm). The difference in the ellipticity of the resulting Ag NPs is summarized in Figure 2. One can see that the deviation of the ellipticity of the resulting Ag NPs is below 10% when the concentration of the residual sodium citrate in the final reaction solution is lower than 0.05 mM, at which the concentrated volume of the final dispersion of AgNP seeds is 500 µL. Therefore, the volumes (Table S1) of the dispersions of Ag-NP seeds used for synthesis of large-sized Ag NPs are concentrated or diluted to 200 µL to guarantee that the impact of residual citrate ions can be completely avoided.

Figure 2. Histogram of the ellipticity of 40 nm Ag NPs prepared by using the same number of AgNP seeds but contained in the different volumes of dispersions: (a) 2 mL, (b) 1 mL, (c) 500 µL, (d) 200 µL, and (e) 100 µL. The final concentrations of the residual sodium citrate in the reaction solution are 0.2 (a), 0.1 (b), 0.05 (c), 0.02 (d) and 0.01 mM (e), respectively. 3.3 Effect of AA Concentration on Synthesis of Ag NPs. Besides the redox potential of AA, the concentration of AA also has an effect on the growth rate at the fixed concentration of silverammonia complex. Thus, a patch of Ag NPs were synthesized by using different concentrations of


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AA at other fixed reaction conditions. As shown in Figure 3 and Figure S4, the optimal AA concentration is in the range of 0.5 to 1 mM as the changes in the ellipticity and the deviation in the size of as-prepared Ag NPs are less than 10%. Accordingly, when AA concentration is beyond the range (too low or too high), the quality of the Ag NPs would become worse (Figure 3). In our case, the concentration of [Ag(NH3)2]+ ions is about 0.43 mM. Accordingly, the molar ratio of optimal AA to silver precursor is in the range of 1.16 to 2.32. Thus, [Ag(NH3)2]+ ions can be totally reduced into silver atoms by AA, since each AA molecule can provide two electrons46 and each silver ion needs one electron theoretically.

Figure 3. Line charts of the evolution in the ellipticity (a) and the deviation in the size (b) of 60 nm Ag NPs obtained at different AA concentrations. The concentrations of AA are 0.1, 0.25, 0.5, 1 and 2 mM, respectively. 3.4 Effect of Final Concentration of Ammonia Water in the Reaction Mixture on Synthesis of Ag NPs. In general, excess ammonia water is used for the preparation of silver-ammonia complex as the excess ammonia water can help stabilize the silver-ammonia complex, and also can further reduce the redox potential of [Ag(NH3)2]+ ions, thereby controlling the growth rate of Ag NPs.36 Thus, a series of Ag NPs were synthesized by using different concentrations of ammonia water at other fixed reaction conditions. As shown in Figure 4 and Figure S5, with the increasing concentration of ammonia water, the ellipticity and the deviation in the size of as-prepared Ag NPs both decrease firstly and then increase when the reaction time is 1 h (solid black lines). When the


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concentration of ammonia water is about 48 mM, the ellipticity and the deviation in the size of asprepared Ag NPs are 1.06 and 7%, respectively. In addition, when the concentration of ammonia water increases to 96 mM, the quality of the resulting Ag NPs becomes a little worse than those obtained at the concentration of ammonia water of about 48 mM. However, their ellipticity and size deviation are still less than 10%. Note that the ellipticity and the deviation in the size of asprepared Ag NPs are over than 10% when the concentration of ammonia water is bigger than 96 mM (solid black lines in Figure 4).

Figure 4. Line charts of the evolution in the ellipticity (a) and the deviation in size (b) of 60 nm Ag NPs obtained at different concentrations of ammonia water. The final concentrations of ammonia water in the reaction mixture are 2.5, 5, 13, 24, 48, 96 and 192 mM, respectively. The solid black lines and dotted red lines represent the quality of the resulting Ag NPs obtained after stirring for 1 h and after 30 min, respectively. The normal reaction time is 1 h for synthesis of Ag NPs. In general, with the increasing concentration of ammonia water, the redox potential of [Ag(NH3)2]+ ions would slightly decrease, thus slowing down the growth rate of Ag NPs and improving their quality. Thus, the occurrence of deterioration in the ellipticity and size deviation is highly due to the fact that the formed Ag NPs can be further etched by ammonia water in the presence of higher concentration of ammonia water.47 In our controlled experiments, 60 nm Ag NPs were still prepared at the concentration of ammonia water bigger than 48 mM, but these Ag NPs were taken out after stirring of 30 min for characterization (dotted red lines in Figure 4 and Figure S6). Surprisingly, the quality of the resulting Ag NPs obtained after reacting of 30 min is


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much better than those obtained after reacting of 1 h, and is also better than those obtained at the lower concentration of ammonia water. Moreover, the evolution of extinction spectra and morphology of 60 nm Ag NPs with reaction times in the presence of the highest concentration of ammonia water (192 mM) was shown in Figure S7. These results demonstrate that high quality Ag NPs indeed can be further etched by ammonia water after their formation if the concentration of ammonia water is bigger than 48 mM. Note that the quality of Ag NPs can keep unchanged within 24 h when the concentration of ammonia water is below 48 mM (Figure S8). Thus, the optimal concentration range of ammonia water for synthesis of Ag NPs in our case is between 24 to 48 mM. On the basis of the kinetic curve of 60 nm Au NPs during the seed-mediated growth reaction with optimized conditions by plotting the variation in their absorbance (A/Amax) of the characteristic peak in the extinction spectra (Figure S9), it is clearly found that the whole growth process of 60 nm Au NPs can be finished within 2 minutes. The reaction solution was further reacted for 1 h under stirring to warrant formation of uniform quasi-spherical Ag NPs. In addition, due to side-effect of ammonia water on the quality of Ag NPs, it is better to remove excess ammonia water in solution by centrifugation and then redisperse these Ag NPs into the aqueous solution of citrate ions for storage and further use. 3.5 Role of Ammonia Water in Synthesis of High Quality Ag NPs. The introduction of ammonia water not only can form silver-ammonia complex with silver ions, but also can impact the pH of the reaction solution. Both roles of ammonia water can affect the redox potential of silver precursor and AA. Which one dominates? When the pH value of the reaction solution was adjusted to the optimal value of 9.6 by NaOH, instead of ammonia water, the quality of the resulting Ag NPs obtained at other fixed reaction conditions was rather poor (Figure S10). This is because silver precursor (AgOH) is not stable, which quickly transforms into Ag2O. Thus, the formation of stable


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silver-ammonia complex is essential for fabrication of high quality Ag NPs, instead of unstable AgOH. Although NH3∙H2O and OH- ions in the reaction system both can complex with Ag+ ions, the formation of AgOH was greatly suppressed.48 This is because the ratio of free OH- ions in the solution is rather small in the absence of additional OH- ions due to the small ionization constant of ammonia water, even at the high concentration of ammonia water. Accordingly, the quality of Ag NPs will not be impacted. In addition, pH values of the reaction solution containing silverammonia complex cannot be further adjusted by additional OH- ions because additional OH- ions would break the ionization balance of ammonia water and further affect the stability of silverammonia complex, thus leading to the formation of AgOH and further deteriorating the quality of Ag NPs (Figure S11). Thus, the optimal concentration of ammonia water was selected to regulate the growing rate of Ag NPs and improve their quality by guaranteeing the formation of silverammonia complex and keep the pH of the reaction media at an optimal pH value. 3.6 Size Control Synthesis of Ag NPs. On the basis of optimal reaction conditions, a series of differently-sized Ag NPs (as shown in Table 1) were prepared by adjusting the particle number of Ag-NP seeds and fixing the amount of silver-ammonia complex. Figure 5 shows typical TEM images of differently-sized, quasi-spherical Ag NPs prepared by one-pot seeded growth method. The average diameters of as-prepared Ag NPs change from 23 ± 2 to 45 ± 4, 60 ± 5, 75 ± 4, 90 ± 6, 100 ± 7, 125 ± 9, 150 ± 11, 200 ± 15, 250 ± 17, and 300 ± 20 nm when the particle numbers (Nsum) of seeds change from 74 × 1010 to 0.22 × 1010 accordingly, at fixed amount of silverammonia complex.


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Figure 5. TEM images (a–j) of differently-sized Ag NPs obtained by adjusting the particle numbers of Ag-NP seeds: 45 ± 4 (a), 60 ± 5 (b), 75 ± 4 (c), 90 ± 6 (d), 100 ± 7 (e), 125 ± 9 (f), 150 ± 11 (g), 200 ± 15 (h), 250 ± 17 (i), and 300 ± 20 nm (j). The insets are their corresponding size distributions obtained by DLS tests. The concentrations of silver precursor, NH3∙H2O and AA are 0.43, 38 and 0.7 mM, respectively. It can be clearly seen that all of as-prepared Ag NPs display a fairly narrow size distribution and a good quasi-spherical shape, which is also in good agreement with their standard deviation in size and good ellipticity, which are below 10% and 1.1, respectively (shown in Table 1). Insets in Figure 5 are the corresponding results from DLS measurements. The resulting Ag NPs are rather colloidally stable in the aqueous solution of sodium citrate after the post-treatment. In addition, their quasi-spherical shapes can be remained for at least 30 days without any change. Although asprepared Ag NPs with diameters over than 75 nm easily precipitate after standing a long time, they are easily dispersed again by shaking. On the basis of the HRTEM images and the selected area electron diffraction (SAED) patterns (Figure S12 and S13), Ag NPs of different sizes were prepared by the arbitrarily deposition of Ag onto the Ag-NP seeds.


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Table 1. Summarized data of particle numbers (Nsum) of Ag-NP seeds used in the growth solution, calculated diameters, real diameters, ellipticities and size deviations of Ag NPs in the corresponding TEM images, the measured and calculated SPR band positions of their extinction spectra. Sample image 5a 5b 5c 5d 5e 5f 5g 5h 5i 5j Nsum [*1010] a) 74 28.7 14.3 8.16 5.91 3.01 1.74 0.73 0.38 0.22 Calculated Diameter 45 60 75 90 100 125 150 200 250 300 [nm] Real Diameter [nm] 45 60 75 90 100 125 150 200 250 300 Ellipticity b) 1.09 1.08 1.08 1.05 1.05 1.06 1.03 1.04 1.04 1.05 Deviation[%] c) 8.89 8.33 5.33 6.67 7.00 7.20 7.33 7.50 6.80 6.67 dipo.f) 418 432 454 476 490 537 596 756 917 none Mie d) quadru.g) nonei) none none 401 405 420 438 494 562 634 h) oct. none none none none none none none 420 456 497 dipo. 416 434 456 480 501 548 607 768 954 none Exp. e) quadru. none none none 396 403 415 436 485 568 652 oct. none none none none none none none 418 452 496 a b The particle number (Nsum) of Ag-NP seeds in the growth solution. Ellipticity is estimated as the ratio of the major to minor axes. c Standard deviation of the Ag NP diameter. d Peak positions of their SPR bands calculated by using Mie theory. e Peak positions of their SPR bands measured by UV−vis spectroscopy. f,g,h Abbreviations for dipolar mode, quadrupolar mode, and octupolar mode (nm), respectively. i None indicates that the corresponding SPR peaks are beyond the test wavelength range (200-1000 nm).

By assuming that (i) all of the [Ag(NH3)2]+ ions in the reaction solution are reduced by AA to deposit onto Ag-NP seeds; (ii) the secondary nucleation for the newly forming sliver atoms doesn’t occur, and (iii) the morphology of the final Ag NPs is spherical, during the classical seeded growth, the size of the final Ag NPs can be precisely calculated30,49 and controlled by using the following Equation (1): 6

D = [ (m + ∆m)/ρN] π

1 3

(1)

where D is the diameter of Ag NPs obtained in theory, π is a constant, m is the total mass of all of additional seeds, ∆m is the extra mass of Ag atoms via the sliver-ammonia complex with the AA reduction, ρ is the density of sterling Ag, N is the particle number (Nsum ) of as-prepared Ag NPs. Figure 6 shows the relationship between the size of the resulting Ag NPs and the number of additional Ag-NP seeds. It is clearly seen that the diameter of Ag NPs is proportional to the cube root of the number of additional Ag-NP seeds, which is in good agreement with Equation (1). The


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results illustrate that the sizes of the synthesized Ag NPs can be accurately controlled by adjusting the particle number of additional Ag-NP seeds at the fixed amount of [Ag(NH3)2]+ ions, which can be predicted by Equation (1).30,49 Thus, the synthesis of differently-sized Ag NPs can be achieved in water via a simple, one-pot seed-mediated growth method.

Figure 6. Relationship between the average size (diameter) of the resulting Ag NPs and the particle number (Nsum) in the growth solution at the fixed amount of silver-ammonia complex. 3.7 Optical Properties of Differently-Sized Ag NPs. The extinction spectra of as-prepared Ag NPs experimentally are shown in Figure 7a. It clearly demonstrates that their optical properties are highly dependent on their average size.50 With the increases in size of Ag NPs, the peak position of their dipolar resonance bands is progressively redshifted to longer wavelengths. For instance, when their size increases from 45 to 60 and 75 nm, the peak position of their dipolar SPR bands is progressively shifted from 416 to 434 and 456 nm, respectively (Table 1, Figure S14a). In addition, these dipolar SPR bands are relatively symmetrical with a narrow half-width, and no nonzero baselines at longer wavelengths are observed (>700 nm). The results illustrate that Ag NPs with sizes in the range of 45 to 75 nm are uniform and bear quasi-spherical shape.21,30 Thus, only one main dipolar SPR band can be observed when the size of Ag NP is smaller than 75 nm.21 Note that


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a rather weak shoulder peak (quadrupole peak) around 390 nm starts to appear in the extinction spectrum of 75 nm Ag NPs (Figure S14a).

Figure 7. Extinction spectra (a) of Ag NPs with different sizes prepared by the seed-mediated growth method. The inset is their optical photos of the aqueous dispersions containing the corresponding Ag NPs. Calculated extinction spectra (b) of the corresponding Ag NPs by Mie theory with the same average sizes and standard deviation rates as those prepared experimentally. Extinction spectra of all of Ag NPs are normalized at 267 nm. Curves 1−10 are the corresponding extinction spectra of Ag NPs with average sizes from 45 to 300 nm, respectively. However, when the size of Ag NPs increases to 90 nm, the new quadrupole peak starts to be clearly observed in the extinction spectrum in addition to the dipole peak and the nonzero baseline at longer wavelengths (>700 nm). When the size of Ag NPs is further increased from 90 to 150 nm, the main dipolar SPR peaks start to clearly broaden and the center positions still redshift from 480 to 607 nm (Figure S14b). The broadening in their SPR peaks is possibly due to the increasing radiative losses of the lager-size Ag NPs.51 Moreover, their quadrupolar SPR peaks progressively become noticeable and the center position also redshifts from 396 to 436 nm and nonzero baselines at longer wavelengths are obviously observed (>700 nm). For the NP size in the range of 90 to 150 nm, there are one main dipolar SPR band and one secondary quadrupolar SPR band in their extinction spectra. With the further increases in size of as-prepared Ag NPs, the center positions of dipolar SPR peaks still redshift along but their SPR peaks become more complicated, which show multiple SPR


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peaks (Figure S14c). In addition, some SPR peaks have extended to the near-infrared wavelength region. As examples, Ag NPs with diameters above 200 nm display apparent octopolar SPR peaks. The positions of octopolar SPR peaks redshift to 418 nm and 452 nm for 200 nm Ag NPs and 250 nm Ag NPs, respectively. In addition, the positions of broader dipolar SPR peaks also redshift to 768 nm and 954 nm for 200 nm Ag NPs and 250 nm Ag NPs, respectively. When the size of Ag NPs is bigger than 300 nm, the octopolar SPR peak evolves as the main SPR peak, which is redshifted to 496 nm and the position of the quadrupolar SPR peaks redshifts to 652 nm at the same time. Moreover, the position of dipolar SPR peaks is redshifted to the near-infrared wavelength region and cannot be observed in the current test range (Figure S14c and Table 1). The colors of the aqueous dispersions containing differently-sized Ag NPs (the inset shown in Figure 7a) also can demonstrate the successive redshift of SPR peaks of as-prepared Ag NPs with their increasing size. The extinction spectra of Ag NPs with the same average sizes and standard deviation rates as those obtained experimentally are calculated by Mie theory, as shown in Figure 7b. Obviously, the extinction spectra of the Ag NPs obtained experimentally are in good consistent with those calculated from Mie theory (Table 1 and Figure 7b). As their extinction spectra are consistent with those obtained by the standard Mie theory of the corresponding spherical Ag NPs, the Ag NPs prepared by our seed-mediated growth method are quasi-spherical and have narrow distributions in size and shape. 3.8 SERS Performance of Differently-sized Ag NPs. The solution-based SERS performances of as-prepared Ag NPs were explored by using R6G as the SERS probe at the excitation laser wavelength of 633 nm (shown in Figure 8a). The intense SERS signals at about 1180, 1310, 1365, 1510 and 1650 cm-1 are attributed to the totally symmetric mode of in-plane C-C stretching vibration. And the peaks at the wavenumbers of about 610 and 775 cm-1 were caused by the C-C-


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C torus vibration and the C-H out-of-plane bending mode, resectively.52 In the basis of the variation in the intensity of SERS band at 1510 cm−1, it can be clearly seen that the SERS activities of as-prepared Ag NPs show a volcano-like curve with their increasing size (shown in Figure 8b). In addition, Ag NPs with size of about 125 nm show the largest SERS activity. Different SERS performance among these Ag NPs might be attributed to the change in their extinction spectra because higher SPR modes appeared with the increase in their NP sizes may further interact together.53–56 The SERS detection limit of monodisperse Ag NPs with an average size of about 125 nm in solution was also investigated (Figure S15). Their detection limits in solution for R6G and crystal violet (CV) can reach to 10-6 M. The successful preparation of large-size Ag NPs may extend their application to the near-infrared region.

Figure 8. Representative SERS spectra (a) of R6G molecules (0.1 mM) on monodisperse Ag NPs (0.01 nM, particle number ) of different sizes in the ethanol solution and line chart (b) of the intensities of SERS signal (at 1510 cm−1) in the SERS spectra of the samples in (a). The laser wavelength is 633 nm, the laser power is 0.425 mW, and the exposure time is 10 s. 3.9 Uniformity and Reproducibility of SERS Signals of Ag NPs in the Solution. The impact of size distribution of Ag NPs on their uniformity and reproducibility of SERS signals in the ethanol solution was studied by taking 45 nm Ag NPs as an example. The representative SERS spectra of R6G molecules adsorbed on 45 nm Ag NPs with size distributions of 8.89% (Figure S16a) and 46.7% (Figure S16b) in the capillary glass tube were conducted. Figure 9 shows the SERS spectra


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of Ag NPs with size distribution of 8.89%, which were obtained by mapping in horizontal direction (Figure 9a) and in vertical direction (Figure 9c) acquisitions with regular spacing (about 10 μm), respectively. Moreover, the histograms of the characteristic SERS band at about 1510 cm-1 in their corresponding SERS spectra obtained by mapping were plotted on the basis of their intensities (Figure 9b and 9d). It can be clearly found that the relative standard deviations (RSDs) in the intensity of the characteristic SERS band at about 1510 cm-1 were 3.67% in horizontal direction (Figure 9b) and 4.81% in vertical direction (Figure 9d), respectively.

Figure 9. SERS spectra (a and c) of 45 nm Ag NPs with size distribution of 8.89% (0.2 nM, particle number concentration) in ethanol solution with R6G (0.1 mM), which were obtained by mapping in horizontal direction (a) and in vertical direction (c) acquisitions with regular spacing (about 10 μm), respectively. The histograms (b and d) of the characteristic SERS band at about 1510 cm-1 in their corresponding SERS spectra obtained by mapping in horizontal direction (b) and in vertical direction (d), which were plotted on the basis of their intensities. The laser wavelength is 633 nm, the laser power is 0.425 mW, and the exposure time is 10 s.


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In contrast, the corresponding RSDs in the intensity of the characteristic SERS band (Figure S17) at about 1510 cm-1 of Ag NPs with size distribution of 46.7% (Figure S16b) were 26.2% in horizontal direction (Figure S17b) and 27.7% in vertical direction (Figure S17d), respectively, which were much higher than those of 45 nm Ag NPs with size distribution of 8.89% (Figure 9b and 9d). The results demonstrated that the narrow size distribution of the Ag NPs in solution is paramount for the uniformity and reproducibility in space scale of their SERS signals. In addition, Ag NPs with size distributions less than 10% can achieve their SERS signals with the RSDs of less than 5% in space scale. The uniformity and reproducibility in temporal scale of SERS signals of 45 nm Ag NPs with size distributions of 8.89% were also investigated (Figure S18). Their RSD in the intensity of the characteristic SERS band at about 1510 cm-1 was about 4.16% during the time range of 30 min (Figure S18). All of the results demonstrate that Ag NPs with narrow distributions in shape and size (say, less than 10%) can achieve the uniformity and reproducibility of their SERS signals in solution; their RSDs can be as low as 5% in space and temporal scale. Thus, the synthesis of Ag NPs with narrow distributions in shape and size can realize their quantitative determination by SERS in solution because the uniformity and reproducibility of their SERS signals in solution can be guaranteed. 4.

CONCLUSIONS

In conclusion, we successfully developed a simple and effective one-pot seed-mediated growth method to prepare uniformly, quasi-spherical Ag NPs with sizes precisely regulated from 40 to 300 nm by using AA reduction of silver-ammonia complex in the aqueous solution with an optimal pH of about 9.6 at room temperature. We found that the quality (narrow distributions in size and shape) of the resulting Ag NPs can be impacted by the residual citrate ions in the Ag-NP seed dispersion, the concentrations of AA and ammonia water. Moreover, the introduction of ammonia


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water can allow us to regulate the growing rate of Ag NPs in the present seeded growth method because ammonia water can form a stable complex with silver precursor and buffer the pH of the reaction solution simultaneously. Furthermore, the average sizes of Ag NPs can be accurately adjusted by changing the amounts of Ag-NP seeds based on the obtained relationship between the NP size and the particle number concentration. Our method can achieve the synthesis of quasispherical Ag NPs with such a large size span (from 40 to 300 nm) in water by one-pot seedmediated growth method without using additional strong stabilizers for the first time, to the best of our knowledge, instead of multiple-step seed-mediated growth methods. Moreover, it can produce high quality quasi-spherical Ag NPs with different sizes and predictable extinction spectra by Mie theory, and thus may allow us to extend these uniform Ag NPs with various sizes to different fields. The as-prepared Ag NPs exhibit size-dependent SERS activity in solution and Ag NPs with size of about 125 nm exhibit the optimal SERS activity at 633 nm excitation wavelength. In addition, our results also demonstrate that Ag NPs with narrow distributions in shape and size (say, less than 10%) can achieve the uniformity and reproducibility of their SERS signals in solution; their RSDs can be as low as 5% in space and temporal scale. Thus, our method for the synthesis of uniform Ag NPs in shape and size can realize their quantitative determination by SERS in solution.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx.


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TEM images of Ag-NP seeds and a series of Ag NPs obtained under different reaction conditions; Additional extinction spectra of Ag-NP seeds and Ag NPs with various sizes; SERS spectra of Ag NPs about 125 nm in ethanol solution containing different concentrations of R6G and CV; SERS spectra in space and temporal scale of 45 nm Ag NPs with different size distributions in ethanol solution with R6G (0.1 mM); summarized recipes for growing Ag NPs with different sizes in Table S1.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (21773142 and 21473105), Taishan Scholarship in Shandong Provinces (No. tsqn20161001), Fundamental Research Funds of Shandong University (2016JC003), and Shandong Provincial Natural Science Foundation for Distinguished Young Scientists (JQ201405).

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Graphical Table of Contents


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Size Control Synthesis of Monodisperse, Quasi-Spherical Silver

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