Abstract
We registered surface enhanced Raman scattering (SERS) spectra of the human lactoferrin molecules adsorbed on a silvered porous silicon (por-Si) from 10−6–10−18 M solutions. It was found that the por-Si template causes a negative surface potential of silver particles and their chemical resistivity to oxidation. These properties provided to attract positively charged lactoferrin molecules and prevent their interaction with metallic particles upon 473 nm laser excitation. The SERS spectra of lactoferrin adsorbed from 10−6 M solution were rather weak but a decrease of the concentration to 10−10 M led to an enormous growth of the SERS signal. This effect took place as oligomers of lactoferrin were broken down to monomeric units while its concentration was reduced. Oligomers are too large for a uniform overlap with electromagnetic field from silver particles. They cannot provide an intensive SERS signal from the top part of the molecules in contrast to monomers that can be completely covered by the electromagnetic field. The SERS spectra of lactoferrin at the 10−14 and 10−16 M concentrations were less intensive and started to change due to increasing contribution from the laser burned molecules. To prevent overheating the analyte molecules on the silvered por-Si were protected with graphene, which allowed the detection of lactoferrin adsorbed from the 10−18 M solution.
Highlights
The development of biosensing techniques to overcome the problem of reliable detection, identification, and structural study of diverse bioorganic molecules at ultralow concentrations is still an urgent objective of specialists in many spheres including medicine, biology, forensics, ecology, Biosensors 2019, 9, 34; doi:10.3390/bios9010034 www.mdpi.com/journal/biosensorsBiosensors 2019, 9, 34 pharmaceutics so on.This is proven by the statistics on “biosensing” papers, the number2 ofof19which 8, x FOR PEERREVIEW has increased greatly in the last years (Figure 1)
It is known that particles of the coinage metals can be deposited on the porous silicon (por-Si) surface because of oxidation of silicon atoms with silver ions from the solution and their following reduction in an atomic form [38,40,56]
The surface enhanced Raman scattering (SERS)-active layer of these substrates presents non-continuous film composed of quasi-spherical silver particles of polycrystalline nature, which dominating sizes are in the range of 10–150 nm, some of them have diameters of 150–700 nm
Summary
The development of biosensing techniques to overcome the problem of reliable detection, identification, and structural study of diverse bioorganic molecules at ultralow concentrations is still an urgent objective of specialists in many spheres including medicine, biology, forensics, ecology, Biosensors 2019, 9, 34; doi:10.3390/bios9010034 www.mdpi.com/journal/biosensorsBiosensors 2019, 9, 34 pharmaceutics so on.This is proven by the statistics on “biosensing” papers, the number ofof19which 8, x FOR PEERREVIEW has increased greatly in the last years (Figure 1). There has been a rapid growth of interest nanomaterials, which are able to find molecules in physiological liquids and living cells at extremely in biosensing studies involving a variety of optical systems, devices and methods combined with tests, low concentrations (Figure 1, inset) [1,2,3,4,5,6,7,8,9,10]. The main goals of these studies include non-invasive nanomaterials, which are able to find molecules in physiological liquids and living cells at extremely medical diagnostics and therapy of different types of cancer [6,8], brain diseases [9], viruses-induced low concentrations (Figure 1, inset) [1,2,3,4,5,6,7,8,9,10]. Accuracy and reliability are important requirements medical diagnostics and therapy of different types of cancer [6,8], brain diseases [9], viruses-induced of desirable to reduce risks false results
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