Sandwiching analytes with structurally diverse plasmonic nanoparticles on paper substrates for surface enhanced Raman spectroscopy

Abstract

This report describes the systematic combination of structurally diverse plasmonic metal nanoparticles (AgNPs, AuNPs, Ag core-Au shell NPs, and anisotropic AuNPs) on flexible paper-based materials to induce signal-enhancing environments for surface enhanced Raman spectroscopy (SERS) applications. The anisotropic AuNP-modified paper exhibits the highest SERS response due to the surface area and the nature of the broad surface plasmon resonance (SPR) neighboring the Raman excitation wavelength. The subsequent addition of a second layer with these four NPs (e.g., sandwich arrangement) leads to the notable increase of the SERS signals by inducing a high probability of electromagnetic field environments associated with the interparticle SPR coupling and hot spots. After examining sixteen total combinations, the highest SERS response is obtained from the second layer with AgNPs on the anisotropic AuNP paper substrate, which allows for a higher calibration sensitivity and wider dynamic range than those of typical AuNP-AuNP arrangement. The variation of the SERS signals is also found to be below 20% based on multiple measurements (both intra-sample and inter-sample). Furthermore, the degree of SERS signal reductions for the sandwiched analytes is notably slow, indicating their increased long-term stability. The optimized combination is then employed in the detection of let-7f microRNA to demonstrate their practicability as SERS substrates. Precisely introducing interparticle coupling and hot spots with readily available plasmonic NPs still allows for the design of inexpensive and practical signal enhancing substrates that are capable of increasing the calibration sensitivity, extending the dynamic range, and lowering the detection limit of various organic and biological molecules.

Fig. 1. SEM and TEM images of the four types of plasmonic NPs on filter paper and their corresponding SPR patterns before and after the loading of NPs using a two-fold diluted solution.

Fig. 2. Digital photos of the four types of plasmonic NPs on filter paper and their corresponding absorption patterns.

Fig. 3. Representative (a) SERS spectra with four different plasmonic papers and (b) their most intense peak wavenumber (average of minimum 9 spots from 3 plasmonic papers under single scan).

Fig. 4. Average SERS signals and EFs before and after applying four different NPs onto (a) AgNP, (b) AuNP, (c) Ag core–Au shell, and (d) anisotropic AuNP plasmonic paper treated with 1 μM of 4-NBT (average of minimum 9 spots from 3 plasmonic papers).

Fig. 5. SERS intensity of 4-NBT as a function of the concentration before and after applying another layer of NPs and their representative SEM images; (a) AuNPs on AuNP plasmonic paper and (b) AgNPs on anisotropic AuNP plasmonic paper.

Fig. 6. Representative (a) SERS spectra of RNA on AuNP and anisotropic AuNP plasmonic paper before and after forming sandwich geometry with AuNPs and AgNPs, respectively, and (b) their average SERS intensities.