Detecting explosive molecules from nanoliter solution: A new paradigm of SERS sensing on hydrophilic photonic crystal biosilica

Abstract

We demonstrate a photonic crystal biosilica surface-enhanced Raman scattering (SERS) substrate based on a diatom frustule with in-situ synthesized silver nanoparticles (Ag NPs) to detect explosive molecules from nanoliter (nL) solution. By integrating high density Ag NPs inside the nanopores of diatom biosilica, which is not achievable by traditional self-assembly techniques, we obtained ultra-high SERS sensitivity due to dual enhancement mechanisms. First, the hybrid plasmonic-photonic crystal biosilica with three dimensional morphologies was obtained by electroless-deposited Ag seeds at nanometer sized diatom frustule surface, which provides high density hot spots as well as strongly coupled optical resonances with the photonic crystal structure of diatom frustules. Second, we discovered that the evaporation-driven microscopic flow combined with the strong hydrophilic surface of diatom frustules is capable of concentrating the analyte molecules, which offers a simple yet effective mechanism to accelerate the mass transport into the SERS substrate. Using the inkjet printing technology, we are able to deliver multiple 100pico-liter (pL) volume droplets with pinpoint accuracy into a single diatom frustule with dimension around 30µm×7µm×5µm, which allows for label-free detection of explosive molecules such as trinitrotoluene (TNT) down to 10^-10M in concentration and 2.7×10^-15g in mass from 120nL solution. Our research illustrates a new paradigm of SERS sensing to detect trace level of chemical compounds from minimum volume of analyte using nature created photonic crystal biosilica materials.

Keywords: Diatom biosilica; Hydrophilic surface; Inkjet printing; Photonic crystal; Surface-enhanced Raman scattering.

Fig. 1

Illustration of in-situ growth of Ag NPs in pores of the diatom frustule with hydrophilic surface and combined with inkjet printing technology for ultrasensitive TNT detection at nanoliter volume scale.

Fig. 2

SEM images of (a) an overview of a single diatom frustule, (b) primary pores on a frustule, (c) deposited Ag seeds on the diatom frustule and Ag NPs in-situ growth from the seeds with (d) 2.5 mM, (e) 5 mM and (e) 10 mM AgNO₃ added in the growth media.

Fig. 3

Raman spectra ofR6G (10⁻⁶ M) on diatom-Ag NPs through in-situ growth at various AgNO₃ concentrations.

Fig. 4

SERS spectra of R6G on diatom with in-situ growth (a) and self-assembled Ag NPs (b) dipped in R6G solutions of different concentrations. SERS intensity as a function of R6G concentrations on diatom with in-situ growth (c) and self-assembled NPs (d).

Fig. 5

SERS spectra of TNT on diatom with in-situ growth Ag NPs dipped in TNT solutions with different concentrations (a) and SERS intensity as a function of TNT concentrations (b).

Fig. 6

SERS spectra of different droplets of TNT at various concentrations cast onto a single diatom frustule with in-situ growth Ag NPs by inkjet printing.