High-Performance Sample Substrate of Gold Nanoparticle Multilayers for Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

Abstract

The development of a sample substrate with superior performance for desorption and ionization of analyte is the key issue to ameliorate the quality of mass spectra for measurements of small molecules in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). Herein, the homogeneous sample substrate of gold nanoparticle multilayers (AuNPs-ML) with hexagonal lattice was successfully prepared by self-assembly technique. With strong surface plasmon resonance absorption and superior photothermal effect, the sample substrate of AuNPs-ML exhibited high signal sensitivity and low background noise for the detection of model analyte of glucose without additional matrixes in SALDI-MS. Furthermore, compared to merchant matrixes of α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB), the sample substrate of AuNPs-ML was demonstrated to ameliorate the quality of mass spectra, including signal strength, background interference and signal/noise (S/N) ratio. The sucrose and tryptophan were also measured to show the extensive applications of AuNPs-ML sample substrate for the detections of small molecules in SALDI-MS. Most importantly, the remarkable reproducibility of glucose mass spectra with relative signal of 7.3% was obtained by the use of AuNPs-ML sample substrate for SALDI-MS. The homogeneous sample substrate of AuNPs-ML greatly improved the quality of mass spectra because of its strong absorption of laser energy, low specific heat, high heat conductivity and extraordinary homogeneity. We believe that AuNPs-ML could be a practical sample substrate for small molecule detection in SALDI-MS.

Keywords: gold nanoparticle multilayers; gold nanoparticles; photothermal effect; reproducibility; sample substrate; self-assembly.

Figure 1

Schematic illustration of self-assembly of gold nanoparticles and fabrication of AuNPs-ML sample substrate. Briefly, stock solution of gold nanoparticles was added dropwise on the surface of deionized water in the trough. Afterward, a cover with a hole at the edge was covered onto the trough. After evaporation for 24 h, the self-assembled gold nanoparticles were obtained on the surface of deionized water. A steel plate was slowly pulled out of the sub-phase to transfer self-assembled gold nanoparticles.

Figure 2

(a) TEM image of gold nanoparticles. (b) Histogram of gold nanoparticle size distributions and their Gaussian fitting. (c) UV-Vis absorption spectra of gold nanoparticle solution.

Figure 3

(a) TEM image of SAM of gold nanoparticles. (b) UV-Vis absorption spectrum of SAM of gold nanoparticles. Contact angles of SAM of gold nanoparticles before (c) and after (d) EDT treatment.

Figure 4

Mass spectra of glucose with the application of AuNPs-ML as the sample substrates in SALDI-MS. The concentration of glucose was 10⁻³ M. Peak identity: m/z 203.26, [Glucose+Na]⁺.

Figure 5

Mass spectra of glucose using AuNPs-ML15 as the sample substrate and merchant matrixes of CHCA and DHB. Peak identity: m/z 203.26, [Glucose+Na]⁺.

Figure 6

Mass spectra of sucrose and tryptophan using AuNPs-ML15 as the sample substrate in SALDI-MS. Peak identity: m/z 343.46, [sucrose+H]⁺; m/z 365.46, [sucrose+Na]⁺; 205.25 m/z, [tryptophan+H]⁺.

Figure 7

The relative signal of glucose was measured and calculated with the sample substrate of AuNPs-ML-15 in SALDI-MS.