Optimization of ZnO Nanorod-Based Surface Enhanced Raman Scattering Substrates for Bio-Applications

Abstract

Nanorods based on ZnO for surface enhanced Raman spectroscopy are promising for the non-invasive and rapid detection of biomarkers and diagnosis of disease. However, optimization of nanorod and coating parameters is essential to their practical application. With the goal of establishing a baseline for early detection in biological applications, gold-coated ZnO nanorods were grown and coated to form porous structures. Prior to gold deposition, the grown nanorods were 30⁻50 nm in diameter and 500⁻600 nm in length. Gold coatings were grown on the nanorod structure to a series of thicknesses between 100 and 300 nm. A gold coating of 200 nm was found to optimize the Rhodamine B model analyte signal, while performance for rat urine depended on the biomarkers to be detected. These results establish design guidelines for future use of Au-ZnO nanorods in the study and early diagnosis of inflammatory diseases.

Keywords: Au coated SERS; ZnO nanorods; bladder disease detection; gold coated thickness; surface enhancement Raman spectroscopy (SERS).

Figure 1

Schematic showing the processes of the ZnO nanorod-based surface-enhanced Raman scattering (SERS) substrate. (a) Au coated substrate production, (b) optimization of application, and (c) measurement of rat urine.

Figure 2

Secondary electron images of the ZnO nanorod substrates show morphology changes based on deposition of different equivalent thickness of gold: (a,f,k) Au: 100 nm; (b,g,l) Au: 150 nm; (c,h,m) Au: 200 nm; (d,i,n) Au: 250 nm; and (e,j,o) Au: 300 nm. (a–e) cross-sectional images show slight increases in thickness with increased gold deposition. (f–j) 45-degree tilted images, and (k–o) plane images show increased filling at higher Au deposition.

Figure 3

(a) SERS spectra and standard deviation (pastel-colored area) of 1 mM Rh B on Au-deposited ZnO nanorods with gold thicknesses of 100 (black curve), 150 (red curve), 200 (blue curve), 250 (green curve), and 300 nm (purple curve). (b) Intensity of SERS peaks as a function of the thickness of Au. (c) Comparison spectrum of the SERS effect with equivalent illumination power for Rh B on Si substrate (black curve) and on ZnO nanorod substrate deposited with 200 nm gold (blue curve), with standard deviation shown (pastel).

Figure 4

SERS spectra of healthy rat urine measured using 100 nm Au (black curve) and 200 nm Au (red curve) substrates. The blue curve shows the corresponding difference in spectra between the 100 nm gold SERS substrate and the 200 nm gold SERS substrate.