Preaggregated Ag Nanoparticles in Dry Swellable Gel Films for Off-the-Shelf Surface-Enhanced Raman Spectroscopy

Abstract

Large, thin (50 μm) dry polymer sheets containing numerous surface-enhanced Raman spectroscopy (SERS) active Ag nanoparticle aggregates have been prepared by drying aqueous mixtures of hydroxyethylcelloulose (HEC) and preaggregated Ag colloid in 10 × 10 cm molds. In these dry films, the particle aggregates are protected from the environment during storage and are easy to handle; for example, they can be cut to size with scissors. When in use, the highly swellable HEC polymer allowed the films to rapidly absorb aqueous analyte solutions while simultaneously releasing the Ag nanoparticle aggregates to interact with the analyte and generate large SERS signals. Either the films could be immersed in the analyte solution or 5 μL droplets were applied to the surface; in the latter method, the local swelling caused the active area to dome upward, but the swollen film remained physically robust and could be handled as required. Importantly, encapsulation and release did not significantly compromise the SERS performance of the colloid; the signals given by the swollen films were similar to the very high signals obtained from the parent citrate-reduced colloid and were an order of magnitude larger than a commercially available nanoparticle substrate. These "Poly-SERS" films retained 70% of their SERS activity after being stored for 1 year in air. The films were sufficiently homogeneous to give a standard deviation of 3.2% in the absolute signal levels obtained from a test analyte, primarily due to the films' ability to suppress "coffee ring" drying marks, which meant that quantitative analysis without an internal standard was possible. The majority of the work used aqueous thiophenol as the test analyte; however, preliminary studies showed that the Poly-SERS films could also be used with nonaqueous solvents and for a range of other analytes including theophylline, a therapeutic drug, at a concentration as low as 1.0 × 10(-5) mol dm(-3) (1.8 mg/dm(3)), well below the sensitivity required for theophylline monitoring where the target range is 10-20 mg/dm(3).

Glass fibre paper-based test strips for sensitive SERS sensing

“…2A). The papers prepared by dropping of the NP solution show the coffee-ring effect, 13,48 which makes it difficult to obtain test strips with a well-dened concentration of NPs per area unit. This leads to a low reproducibility of the coating and strong changes in the SERS signal.…”

Surface‐Enhanced Raman Spectroscopy as a Probe of the Surface Chemistry of Nanostructured Materials

“…For example, aggregated silver colloids which are held within a dry swellable polymer (hydroxyethylcellulose) film have been prepared as off-the-shelf SERS sensors. [20] Their principle of operation is that in the dry state the aggregates are protected within the host polymer but addition of aqueous analyte solution causes the films to swell, which releases the particles to interact with the target analytes and pulls the aqueous solution into the film to promote the interaction. In these films, citrate-reduced silver colloids are used in the preparation and aggregation is carried out using non-coordinating salts but in situ SERS measurements show that the active surfaces in the film carry an unintended surface chloride layer which is accidently created by ligand displacement during the film preparation step.…”

“…[20,21] Again, detecting the nature of the surface species on metal nanoparticles buried within a polymer host would be extremely challenging by any other analytical method but is extremely straightforward with SERS.…”

Surface-enhanced Raman spectroscopy of novel psychoactive substances using polymer-stabilized Ag nanoparticle aggregates

“…These Poly-SERS films provide a convenient and rapid method for either occasional SERS analysis at scenes or routine on-site screening of suspect drugs of abuse which bridges the gap between highly sensitive but time consuming methods such as GC-MS and low specificity and sensitivity methods, such as colorimetric tests. Initial experiments on seized drugs were carried out under approximately the same conditions as were previously developed for pure pharmaceuticals 31 and other analytes such as thiophenol 30 , where droplets of aqueous analytes were used. Briefly, 15 µL aqueous droplets were deposited onto Ag (CRSC, citrate-reduced silver colloid) Poly-SERS films and allowed to dry before being Raman probed using a Raman microscope.…”

“…The sampling error would be expected to be the larger of the two, since the transfer process depends on collecting and the depositing the same proportion of the sample each time which is clearly prone to errors while the Poly-SERS films have previously been shown to have very good enhancement reproducibility (3.2 % S.D). 30 Of course, as for any SERS technique an internal standard could be added to normalise the SERS response of the substrate but here it is not appropriate since this is not the dominant source of error and because the objective here was to develop a field screening test rather than a quantitative analytical method. Finally, a key aspect of this approach is the shelf life of the enhancing materials, since the objective was to develop an approach which would allow SERS analysis to be available and used as required, even by users who only rarely make use of the technique and who would not be in position to prepare fresh enhancing materials every time the need arose.…”