The role of adatoms in chloride-activated colloidal silver nanoparticles for surface-enhanced Raman scattering enhancement

Chloride-capped silver nanoparticles (Cl-AgNPs) allow for high-intensity surface-enhanced Raman scattering (SERS) spectra of cationic molecules to be obtained (even at nanomolar concentration) and may also play a key role in understanding some fundamental principles behind SERS. In this study, we describe a fast (<10 min) and simple protocol for obtaining highly SERS-active colloidal silver nanoparticles (AgNPs) with a mean diameter of 36 nm by photoconversion from AgCl precursor microparticles in the absence of any organic reducing or capping agent. The resulting AgNPs are already SERS-activated by the Cl− ions chemisorbed onto the metal surface where the chloride concentration in the colloidal solution is 10−2 M. Consequently, the enhanced SERS spectra of cationic dyes (e.g., crystal violet or 9-aminoacridine) demonstrate the advantages of Cl-AgNPs compared to the as-synthesized AgNPs obtained by standard Ag+ reduction with hydroxylamine (hya-AgNPS) or citrate (cit-AgNPs). The results of SERS experiments on anionic and cationic test molecules comparing Cl-AgNPs, hya-AgNPs and cit-AgNPs colloids activated with different amounts of Cl− and/or cations such as Ag+, Mg2+ or Ca2+ can be explained within the understanding of the adatom model – the chemisorption of cationic analytes onto the metal surface is mediated by the Cl− ions, whereas ions like Ag+, Mg2+ or Ca2+ mediate the electronic coupling of anionic species to the silver metal surface. Moreover, the SERS effect is switched on only after the electronic coupling of the adsorbate to the silver surface at SERS-active sites. The experiments presented in this study highlight the SERS-activating role played by ions such as Cl−, Ag+, Mg2+ or Ca2+, which is a process that seems to prevail over the Raman enhancement due to nanoparticle aggregation.

 UV-Vis absorption of the individual reagent solutions used for the preparation of Cl-AgNPs  UV-Vis spectra obtained during the Cl-AgNPs photosynthesis, without addition of hydrogen peroxide and evidence of SERS activity of Cl-AgNPs  Concentration dependent SERS spectra of R6G  Specific SERS detection

Concentration dependent SERS spectra of R6G
In the case of R6G analysed with as-synthesized Cl-AgNPs, intense SERS spectra of R6G could be obtained at a concentration of 10 -7 M ( Figure  show the raw spectra, as recorded. The SERS spectrum of R6G 10 -8 M acquired with as-synthesized Cl-AgNPs ( Figure   S3 A, spectrum b) is dominated by a fluorescence background, due to the 'free', unadsorbed molecules. However, the addition of Ca(NO 3 ) 2 10 -4 M promotes the adsorption of additional Clions, which form SERS active sites for R6G molecules.
Consequently, the SERS spectrum of R6G 10 -8 M becomes more intense, while the fluorescence background becomes much lower ( Figure S3 A, spectrum c).
When analysing R6G in a concentration of 10 -9 ( Figure Figure S3 D, spectrum c). Therefore, these results suggest that for R6G in a concentration below 10 -10 M, Ca 2+ 0.1 mM is enough for the chemisorption all R6G S6 molecules, leading to a sensitivity of SERS spectroscopy which is comparable to that of fluorescence.
The SERS spectrum of R6G shows only fluorescence emission when using assynthesized cit-AgNPs as substrate ( Figure S4). However the SERS spectrum of R6G is turned on by additional activation of the colloid with Ca 2+ and Cl -.

Specific SERS detection
The spectral table depicted in Figure S5 contains spectra recorded from the same R6G 10 -11 M in cit-AgNPs solution, modified by sequentially adding Ca 2+ and Cl -, as shown in the Scheme S1. Scheme S1. Schematic illustration for the sequential addition of Ca(NO 3 ) 2 0.1 mM and NaCl 1 mM and 10 mM to R6G 10 -11 M in cit-AgNPs, as indicated. Figure S5 shows the SERS spectra recorded in each of the situations depicted in Scheme S1. The SERS spectrum of R6G 10 -11 M with cit-AgNPs is blank ( Figure S5, spectrum a).
When to the same solution, Ca(NO 3 ) 2 10 -4 M is added, the recorded spectrum shows the two main citrate SERS bands at 924 and 1370 cm -1 ( Figure S5, spectrum b), S9 indicating that the Ca 2+ SERS active sites promote the chemisorption of citrate anions.
Next, when NaCl 10 -3 M is added to the same solution, the SERS bands of citrate disappear, while the SERS bands of R6G begin to appear ( Figure S5, spectrum c).
Thus, the Clions, due to their higher affinity for the silver surface, replace the citrate anions from the Ca 2+ SERS active sites, forming now Cl -SERS active sites for R6G.
Finally, the increase in the Clconcentration from 10 -3 to 10 -2 M leads to a further increase in the SERS intensity of R6G ( Figure S5 The SERS spectrum of crystal violet 10 -8 M with cit-AgNPs is blank (Figure S6 A, spectrum a). The addition of Ca 2+ 10 -4 M turns on the SERS spectrum citrate ( Figure   S6 A, spectrum b). Next, the addition of Cl -10 -3 M determines the replacement of the citrate anions by Clanions at the Ca 2+ SERS active sites, forming thus Cl -SERS active sites for the cationic dye. Consequently, intense SERS spectra of crystal violet are recorded ( Figure S6 A and B, spectrum c). Moreover, increasing Clconcentration up to 10 -2 M leads to a further increase in the SERS intensity of crystal violet (Figure S6 B, spectrum d). Figure S7 illustrates schematically the obtaining of the SERS spectra depicted in Figure S6, particularly the generation of Ca 2+ SERS active sites, the chemisorption of S11 citrate and the generation of Cl -SERS active sites, which then promote the chemisorption of crystal violet. Figure S7. Schematic illustration for the obtaining of the SERS spectra depicted in Figure S6, after the SERS activation of the cit-AgNPs with Ca 2+ and Clions.
In the absence of Ca 2+ and Clions, there is no electronic contact between the silver surface and citrate or crystal violet molecules, the resulting Raman spectrum being similar to that of water.
The addition of Ca 2+ promotes the chemisorption of citrate anions, thus the SERS spectrum of citrate was obtained.
Further addition of Clions determines the replacement of citrate from the Ca 2+ SERS active sites by Cl -, due to their higher affinity for the silver surface. The Clions form SERS active sites for crystal violet cationic dye, thus the SERS spectrum crystal violet is obtained.