Tuning Nanographene-Enhanced Raman Scattering for Rapid Label-Free Detection of Amino Acids

Abstract

The rapid and sensitive detection of amino acids is important not only for fundamental studies but also for the establishment of a healthy society. However, conventional detection methods have been hampered by the difficulties of low sensitivity, long sampling and detection times, and expensive operation and instruments. Here, we report the plasma engineering of bioresource-derived graphene quantum dots (GQDs) as surface-enhanced Raman scattering (SERS)-active materials for the rapid and sensitive detection of amino acids. Surface-functionalized GQDs with tuned structures and band gaps were synthesized from earth-abundant bioresources by using reactive microplasmas under ambient conditions. Detailed microscopy and spectroscopy studies indicate that the SERS properties of the synthesized GQDs can be tuned by controlling the band gaps of synthesized GQDs. The plasma-synthesized metal-free GQDs with surface functionalities showed improved SERS properties for rapid amino acid detection with low detection limits of 10^-5 M for tyrosine and phenylalanine. Theoretical calculations suggest that charge transfer between GQDs and amino acids can enhance the SERS response of the GQDs. Our work provides insights into the controlled engineering of SERS-active nanographene-based materials using the plasma-enhanced method.

Keywords: Raman scattering; amino acids; band gap tuning; biosensors; graphene quantum dots; plasmas.

Scheme 1. Plasma Engineering of Bioresource-Derived GQDs as the Precious Metal-Free SERS-Active Materials for Rapid and Sensitive Detection of Amino Acids

Surface-functionalized GQDs with controlled structures and band gaps were initially synthesized from earth-abundant bioresources using reactive microplasmas under ambient conditions. The probe amino acids were attracted onto the surfaces of synthesized GQDs by the π–π* interaction. Charge transfer (CT) between GQDs and amino acids is found to be the factor enhancing the Raman scattering. The plasma-synthesized metal-free GQDs show enhanced Raman scattering. The plasma-synthesized metal-free GQDs show enhanced SERS properties for rapid and sensitive amino acid detection.

Figure 1

Plasma synthesis and absorbance study of bioresource-derived GQDs. (a) Schematic illustration of GQD synthesis from bioresources. (b) Visual representation of the GQD solution after microplasma treatment under both daylight and UV-light conditions. Absorption spectra of (c) CA-derived GQDs, (d) F-derived GQDs, (e) L-derived GQDs, (f) CS-derived GQDs, and (g) S-derived GQDs.

Figure 2

In situ absorption spectroscopy and OES study of GQD synthesis. (a) In situ absorbance for CS-GQD, F-GQD, and CA-GQD with different times. (b–d) Different sections of the OES spectra of GQD synthesis from different precursors using microplasmas.

Figure 3

Photoluminescence spectroscopy and UPS study of plasma-synthesized GQDs. Photoluminescence (PL) maps of (a) CA-GQD, (b) F-GQD, (c) L-GQD, (d) CS-GQD, and (e) S-GQD. UPS spectra of (f) CA-GQD, (g) F-GQD, (h) L-GQD, (i) CS-GQD, and (j) S-GQD.

Figure 4

TEM study of the plasma-synthesized GQDs. TEM images of (a) L-GQD, (b) CS-GQD, (c) F-GQD, (d) S-GQD, and (e) CA-GQD. Particle size distributions of (f) L-GQD, (g) CS-GQD, (h) F-GQD, (i) S-GQD, and (j) CA-GQD.

Figure 5

XPS analysis of plasma-synthesized GQDs. High-resolution XPS spectra of (a) C 1s, (b) O 1s, and (c) N 1s of CS-GQD. GQD bonding percentages of (d) C 1s, (e) O 1s, and (f) N 1s from the XPS data.

Figure 6

Raman scattering and FRET study of plasma-synthesized GQDs. (a) SERS spectra of R6G with different plasma-synthesized GQDs. (b) Raman intensity of R6G at 611 cm^–1 with different plasma-synthesized GQDs. (c) SERS spectra for R6G with L-GQD. (d) Schematic of charge transfer between the GQD and analyte. (e) Normalized absorbance spectrum of RG6a and PL spectrum of CS-GQD under 450 nm excitation. The overlapped area indicates the FRET effect between the CS-GQD and R6G. (f) Overlap area percentage of FRET between GQDs synthesized from different precursors and R6G.

Figure 7

Raman spectra of amino acids and selectivity of amino acid detection for plasma-synthesized GQDs. (a) Raman spectra of the amino acid powders. The major Raman peaks of amino acids are shown. (b) Selectivity maps of amino acids with different bioresource-derived GQDs. The Raman intensities are analyzed from the major.

Figure 8

SERS-based detection of amino acids with different plasma-synthesized GQDs. SERS spectra of (a) Tyr with L-GQD, (b) Trp with L-GQD, (c) Glu acid with CS-GQD, (d) Phen with S-GQD, (e) Gly with S-GQD, and (f) Cys with F-GQD.