Microfluidic approach for in situ synthesis of nanoporous silver microstructures as on-chip SERS substrates

Nanoporous silver microstructures (np-AgMSs) were first in situ synthesized using a microfluidic system. Chloride induced precipitation with NH4OH was used to prepare AgCl as a sacrificial template. Exploiting the microfluidic platform, effects of reactant concentrations on the morphology of AgCl templates were studied by simply adjusting the input flow rates. The AgCl templates were subsequently reduced into Ag0 or np-AgMSs using chemical reduction. The synthesized np-AgMSs were tested as on-chip SERS substrates using p-aminothiophenol as an analyte. It was found that the np-AgMSs with multipod structures (tripod and tetrapod) provided the highest SERS signal compared to other structures. Finally, the multipod np-AgMSs were successfully used as on-chip SERS substrates for determination of thiocyanate in human saliva. Linearity of the proposed approach was found to be in the range of 1-20 μM with R2 = 0.9887. The LOD and LOQ were 0.1 and 1.5 μM, respectively. Accuracy of the system was investigated from % recovery of spiked thiocyanate standard solutions and found to be in the range of 90-110%. In addition, reproducible Raman signals of thiocyanate were achieved with relative standard deviations of less than 9% for both intraday and interday SERS measurements. These indicated high accuracy and precision of the whole system.