A drop of aqueous suspension containing PEG-coated AgNPs was deposited on carbon-coated Cu grid. Excess water was remove by filter paper, and then the sample was left to dry under ambient air. SERS spectra were recorded using Advantage 532 and Advantage 200A Raman spectrometers (DeltaNu, Laramie, WY, USA) equipped with a double frequency NdYAG laser emitting at 532 nm (5-mW laser power) and a HeNe laser emitting at 632.8 nm (4-mW laser power), respectively. The spectral resolution of the two spectrometers was 10 cm−1. All the SERS spectra were recorded in 1-ml glass vials filled with 475 μl of silver colloid and 25 μl of analyte. Accumulation

times between 0.1 to 40 s were used, the final spectrum being the average of previous four recordings. Results and discussion PEG-reduced silver colloids PEG 200 (600 μl) and NaOH 1% (500 μl) learn more were added to 90 ml of water in an Erlenmeyer glass and heated to boil on a magnetic stirrer with heating option. A 10-ml aqueous solution containing

0.017 g AgNO3 was then added rapidly or dropwise using a syringe, under vigorous stirring. The formation of AgNPs started immediately, as Obeticholic Acid proven by a significant color shift of the solution towards a light yellow, thus suggesting that the chemical reaction took place and that the seeds are available in the solution. The UV–vis spectra recorded on a sample taken straight Digestive enzyme after the color shift exhibit a peak located close to 400 nm, thus providing the existence of PEG-reduced AgNPs in the solution. The pH right after preparation was 8, but in time, a slight lowering of the pH was observed. Several days after preparation (at the moment when the SERS spectra were recorded), the pH of the PEG-reduced colloid was 7.5. Several colloids have been prepared using different PEG 200 volumes between 340 and 680 μl. All colloids were found to be SERS active. A volume of 600 μl PEG 200 was found to be an optimum in terms of time stability and SERS enhancement. The calculated

molar concentration of the PEG-coated AgNPs was 4 × 10−9 M [16]. Hydroxylamine-reduced silver colloids Briefly, 0.017 g of silver nitrate was solved in 90 ml of water. In a separate recipient, 0.017 g of hydroxylamine hydrochloride was solved in 10 ml of water, followed by the addition of 1.150 ml 1% sodium hydroxide solution. The hydroxylamine/sodium hydroxide solution was then added rapidly to the silver nitrate solution under vigorous stirring. After a few seconds, a gray-brown colloidal solution was produced, which was further stirred for 10 min. The pH value of the silver colloid, measured immediately after preparation, was found to be 8. Also, a slight lowering of the pH was observed, i.e., at measuring time, the pH was 7.5 [9]. Citrate-reduced silver colloids Lee-Meisel method was employed in order to prepare citrate-reduced silver colloids [7].