A Filter-based Surface Enhanced Raman Spectroscopic Assay for Rapid Detection of Chemical Contaminants

Summary

A procedure for fabrication and performing the filter-based surface enhanced Raman spectroscopic (SERS) assay for the detection of chemical contaminants (i.e., pesticide ferbam and antibiotic ampicillin) is presented.

Abstract

We demonstrate a method to fabricate highly sensitive surface-enhanced Raman spectroscopic (SERS) substrates using a filter syringe system that can be applied to the detection of various chemical contaminants. Silver nanoparticles (Ag NPs) are synthesized via reduction of silver nitrate by sodium citrate. Then the NPs are aggregated by sodium chloride to form nanoclusters that could be trapped in the pores of the filter membrane. A syringe is connected to the filter holder, with a filter membrane inside. By loading the nanoclusters into the syringe and passing through the membrane, the liquid goes through the membrane but not the nanoclusters, forming a SERS-active membrane. When testing the analyte, the liquid sample is loaded into the syringe and flowed through the Ag NPs coated membrane. The analyte binds and concentrates on the Ag NPs coated membrane. Then the membrane is detached from the filter holder, air dried and measured by a Raman instrument. Here we present the study of the volume effect of Ag NPs and sample on the detection sensitivity as well as the detection of 10 ppb ferbam and 1 ppm ampicillin using the developed assay.

Introduction

Surface enhanced Raman spectroscopy (SERS) is a technique combining Raman spectroscopy with nanotechnology. The intensity of Raman scattering of analytes at noble metallic nano-surfaces is greatly enhanced by the localized surface plasmon resonance.¹ Silver nanoparticles (Ag NPs) are by far the most widely used SERS substrates due to its high enhancement ability.² Up to now, various synthetic methods of Ag NPs have been developed.^3-6 Ag NPs can be used alone as effective SERS substrates, or combined with other materials and structures to enhance its sensitivity and/or functionality.^7-11

SERS techn....

Protocol

1. Silver Nanoparticle Synthesis¹⁵

1. Dissolve 18 mg silver nitrate in 100 ml ultrapure water (18.2 ΩU) and vortex for 5 sec.
2. Dissolve 27 mg sodium citrate dihydrate in 1 ml water and vortex for 5 sec.
3. Transfer all of the prepared silver nitrate solution to a conical flask containing a stirring bar and put the flask on a magnetic hot plate. Heat the flask under vigorous stirring with a stirring speed of 700 rpm at ~350 °C (setting temperature on the plate).
4. When boiling, add all of the prepared sodium citrate solution to the conical flask immediately, and leave the solution to boil for an additional 25 min un....

Results

The major steps of this experiment were shown in the schematic diagram (Figure 1). Figure 2 demonstrated the importance to use the optimized volume of AgNPs in the membrane coating in order to reach the maximized sensitivity. 1 ml of Ag NPs provides the strongest signal when using ferbam, as compared to 0.5 ml (insufficient coating) or 2 ml (too much coating).

We were able to detect ferbam at 10.......

Discussion

One of the critical steps in this protocol is the Ag NPs synthesis, where uniform Ag NPs are the key for consistent results. The heating time and the concentrations of precursors must be precisely controlled. The average size of this AgNPs preparation is 80 nm, which was measured by the Zetasizer (data not shown). Another critical step is the salt aggregation where the salt concentration and aggregation time must be precisely controlled. In addition, the choice of membrane is also critical as the membrane with a smaller .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ampicillin	Fisher Scientific	BP1760-5	N/A
Ferbam	Chem Service	N-11970-250MG	98+%
Silver nitrate	Sigma Aldrich	209139	99.0+%
Sodium citrate dehydrate	Sigma Aldrich	W302600	99+%
Sodium chloride	Sigma Aldrich	S7653	99.5+%
EMD Millipore Durapore PVDF Membrane Filters	Fisher Scientific	VVLP01300	0.10 µm Pore Size, hydrophilic
Polycarbonate Filter Holders	Cole-Parmer	EW-29550-40	13 mm diameter
Analog Vortex Mixer	Fisher Scientific	02-215-365	N/A
Nutating Mixers	Fisher Scientific	05-450-213	N/A
DXR Raman spectroscope	Thermo Scientific	IQLAADGABFFAHCMAPB	Laser power: 1 mW Exposure time: 5 sec