| dc.description.abstract |
Today, silver nanoparticles (AgNPs) are being widely used in consumer products, water disinfectants, therapeutics, biomedical devices and surface-enhanced Raman spectroscopy (SERS)-based sensing. AgNP size and aggregation state greatly influence these applications. However, many challenges arise from the toxicity of reagents, high costs or reduced efficiency of the AgNP synthesis or isolation methods (e.g., centrifugation, size-dependent solubility, etc.). Tangential flow ultrafiltration (TFU) is a recirculation method that passes a liquid sample through a series of hollow fiber membranes with pore size ranging from 1,000 kD to 10 kD in order to isolate proteins or cells according to their weight. In this study, TFU was modified to size- select and significantly concentrate a large volume of polydisperse colloidal AgNPs. The well- established Creighton method was utilized to synthesize unfuctionalized AgNPs (4 L, 14.7 µg/mL) by the reduction of AgNO3 with NaBH4. AgNP polydispersity was decreased through a 3- step TFU process using a 50 nm filter to remove AgNPs and AgNP-aggregates larger than 50 nm followed by two 30 kD filters to concentrate the AgNPs. TFU may be considered a “green” method as it neither damages the sample nor requires additional solvent to eliminate toxic excess reagents and byproducts. Representative TFU samples were characterized using TEM, UV-Vis absorption spectrophotometry, Raman spectroscopy and ICP-OES. The final 30 kD retentate of AgNPs was highly concentrated (2 mL, 6.04 × 10^2 µg/mL of silver) but minimally aggregated and homogeneous (AgNPs of 1-20 nm diameter). The highly concentrated AgNPs were found to greatly enhance the SERS-based sensing capabilities of the Creighton colloid. SERS events were observed at 10^-9 M of R6G as compared to the original AgNP colloid detection limit of 10^-6 M of R6G. This was attributed to an increased number of SERS hot spots available for a target molecule within the minimal focal volume. |
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