Surface enhancement Raman scattering (SERS) is a nonlinear optical enhancement produced on the rough surface in nanometer scale. It can provide the unique vibration spectrum signal for molecules and has been widely used in biology, chemistry, medicine, physics and environmental monitoring. A better SERS active substrate has the characteristics of strong enhancement, good uniformity, easy preparation and storage, and convenient use. Through the design and construction of nanostructures with SERS activity, it can be used for rapid and sensitive detection research. Nano material with periodic pore structure, such as nanobowl, with special morphology, overcomes shortcomings of agglomeration of nanoparticles. Nanobowl not only has more "hot spots" which can enhance the electric field around greatly, resulted in enhanced SERS effect, but also has good stability and biocompatibility. Nanobowl arrays can strengthen SERS signals, it is an important substrate for realizing biological sensing detection via SERS. In addition, the metal nanobowl arrays with good stability and biocompatibility is very suitable for biomedical research and has wide application prospect. In this work, we constructed ordered nanobowl arrays which used as SERS active substrates. By screening reporters with distinguished SERS peaks, SERS technique was design for detection of one or two cancer markers based on ordered nanobowl array.
The work has been carried out as followings.
1. Preparation of SiO2 colloidal crystal film with high quality
Using vertical self-assembly method, driven by the capillary force, monodisperse SiO2 microspheres assemble into high quality, large areas of SiO2 colloidal crystal film. The microscopic morphology and optical properties of SiO2 colloidal crystal film have been characterized by the scanning electron microscope (SEM), UV-vis spectrophotometer. The colloidal crystal film made of SiO2 microspheres with diameter of 190 nm has close-packed array with bright color. The thickness of SiO2 colloidal crystals was measured by the thin film analyzer. The results were in accordance with the thickness measured by the scanning electron microscope. The preparation of high quality SiO2 colloidal crystals film lay the foundation for fabrication of ordered nanobowl arrays.
2. Fabrication of ordered gold nanobowl arrays.
Firstly, SiO2 arrays were modified by APTES, which then immersed into gold nanoparticles solution to form SiO2/GNPs. The gold nanoparticles that adsorbed on the surface of SiO2/GNPs arrays were used as nucleation sites to template the growth of a gold nanoshells layer by H2O2, which reduce AuCl4- to Au0. With excessive growth solution, uniform gold nanoshell arrays formed. Finally, a monolayer of hexagonal close-packed gold nanoshell arrays was lifted up by tape from the gold nanoshell arrays, and the silica spheres were removed by hydrofluoric acid, producing a monolayer ordered gold nanobowl arrays adsorbed on the tape. Ordered gold nanobowl arrays were characterized by scanning electron microscopy (SEM) and confocal Raman spectrometer, and they showed strong SERS effect. The ordered gold nanobowl arrays also have uniform morphology, good repeatability with rich "hot spots" and exhibits excellent SERS enhancement performance. In addition, the periodic hole in the ordered gold nanobowl arrays can enlarge specific surface area, which could facilitate the immobilization of biomolecule.
3. Construction of ordered gold/silver bimetallic nanobowl arrays.
Based on ordered gold nanobowl arrays, the ordered gold/silver bimetallic nanobowl arrays were prepared. At first, gold nanoparticles were absorb on the SiO2 colloidal crystal film via electrostatic adsorption to fabricated ordered gold shell arrays with crack, which then coated by silver through sputter deposition. Finally, the ordered gold/silver bimetallic nanobowl arrays were prepared by etching SiO2 colloidal crystal film. Because the gold nanomaterial has high stability, silver nanomaterials possess stong SERS enhancement effect, the prepared ordered gold/silver bimetallic nanobowl arrays with both advantages of good stability and strong SERS enhancement effect. It can be seen that ordered gold/silver bimetallic nanobowl arrays is an SERS active substrate, and it was explored to detect 6-MP with high sensitivity, the detection limit is 10-10 M. Ordered gold/silver bimetallic nanobowl arrays can also be applied in detection of other biomolecules. What’s more, it show potential in biological sensing, detection and imaging.
4. Cancer biomarker detection based on ordered gold nanobowl arrays
Based on the ordered gold nanobowl arrays, a new method for detecting cancer biomarker (CEA and AFP) has been developed. The ordered gold nanobowl arrays and the gold nanoshells (gold nanostars) were used as the capture substrate and labeling tags, respectively. In the presence of antigen, capture substrate and labeling tags formed sandwich structures. The concentration of cancer biomarker is quantified by detecting the strength of the SERS signal. This technique can not only detect single cancer biomarker, but also detect two cancer biomarkers simultaneously. Due to the large specific surface area of ordered gold nanobowl arrays, the antibody loading capacity can be improved effectively. The results showed that this SERS method is simple with high sensitivity and strong interference resistance, which can be used for the analysis and detection of cancer biomarker.