Fabrication Of Sandwich Silicon-Based SERS Chips And Their Applications For Biochemical Analysis

Surface-enhanced Raman scattering(SERS)spectroscopy has been widely employed in the biological and chemical fields owing to its excellent specificity,high sensitivity,anti-fluorescence interference,good stability and so on.Among various kinds of SERS substrates,silicon nanohybrids-based SERS substrates have attracted much attention ascribed to their distinct SERS enhancement effect and good signal reproducibility.To meet the requirements of analysis in different fields,silicon-based SERS substrates have been further functionalized to construct SERS sensors with different analytical capabilities.Herein,two categories of sandwich silicon SERS chips are designed and fabricated:(1)graphene(G)-silver nanoparticles(AgNPs)-silicon(Si)sandwich nanohybrids substrate(G@AgNPs@Si);(2)zeolitic imidazolate framework-8(ZIF-8)-silver nanoparticles(AgNPs)-silicon(Si)sandwich nanohybrids substrate(ZIF-8@AgNPs@Si).The as-prepared G@AgNPs@Si chip is suitable for the molecular and cellular analysis and the developed ZIF-8@AgNPs@Si chip is available for quantification of gas.The details are as follows:In the first part,we develop a kind of SERS substrate,made of graphene(G)-silver nanoparticles(AgNPs)-silicon(Si)sandwich nanohybrids substrate(G@AgNPs@Si),in which AgNPs are in situ grown on a silicon wafer through hydrofluoric acid-etching-assisted chemical reduction,followed by coating with single-layer graphene via a wet transfer method.The as-prepared substrate has the following merits:(1)strong SERS enhancement attributed to synergistic effects of three kinds of resonances:silicon-reflected plasmon resonance,AgNPs-scattered plasmon resonance,and graphene-induced charge-transfer resonance;(2)good antioxidation and corrosion resistance abilities;(3)2D peak of graphene located in the Raman silent zone,which can be employed as the desirable internal standard to correct SERS intensities of target molecules;(4)capability for the molecular and cellular analysis.At the molecular level,the fluorescent dye(Rhodamine X,ROX)labeled adenosine triphosphate(ATP)aptamers are adsorbed physically by graphene through ?-? interaction with strong signal intensity(signal on).Upon interaction with ATP,the conformation of the aptamers would change into stable,internal loop structures,weakening the binding effect between the aptamers and graphene and triggering the release of the aptamers from SERS substrate(signal off).In addition,this SERS sensor allows reliable detection of ATP with a detection limit of?1 pM.At the cellular level,vancomycin(Van)is employed to modify G@AgNPs@Si for capture,discrimination,and inactivation of bacteria.Van/G@AgNPs@Si can capture bacteria via hydrogen bonds between the carbonyl and amine groups of vancomycin and peptidoglycan on the bacterial cell wall.The differences in the SERS spectra can be used for discriminating among different kinds of bacteria.Vancomycin,graphene and silver nanoparticles have distinct antibacterial effect,thus leading to the good antibacterial ability of Van/G@AgNPs@Si.The capture rates of Van/G@AgNPs@Si for Staphylococcus aureus(S.aureus)and Escherichia coli(E.coli)are 54%and 39%,respectively,and the antibacterial efficiencies are 93%and 85%,respectively.In the second part,ZIF-8 can be in situ self-assembled on the surface of AgNPs@Si(ZIF-8@AgNPs@Si)by utilizing zinc nitrate hexahydrate and 2-methylimidazole as precursors.The as-prepared substrate features several merits:(1)sensing gaseous molecules owing to the enrichment of gaseous molecules via the coated ZIF-8;(2)displaying good reproducibility in the detection of volatile organic compounds(the relative standard deviation(RSD)<14%)due to the uniform SERS"hot spots" on the SERS substrates;(3)realizing quantitative analysis of gaseous benzaldehyde with the limit of detection(LOD)down to-59 nM when phenylacetylene as internal standard(IS)molecules is modified on AgNPs@Si.In summary,silicon-based SERS substrates(AgNPs@Si)are functionalized to construct two categories of sandwich silicon SERS chips.Such resultant graphene-silver nanoparticles-silicon sandwich nanohybrids substrate(G@AgNPs@Si)can be employed for ATP and bacterial detection,and the developed zeolitic imidazolate framework-8-silver nanoparticles-silicon sandwich nanohybrids substrate(ZIF-8@AgNPs@Si)can be applied to detect volatile organic compounds.