Petr I. Nikitin

Phase jumps and interferometric surface plasmon resonance imaging

Conditions at which phase of light demonstrates a markedly different behavior for close values of system parameters as well as the Heaviside jump are discussed and explained in terms of phase topology. On this basis, a method of interferometric surface plasmon resonance imaging is proposed and applied to develop ultrasensitive affinity array sensors with a monoatomic thickness resolution.

Surface plasmon resonance interferometry for biological and chemical sensing

Abstract Surface plasmon resonance (SPR) interferometry is reported as a novel technique for biological and chemical sensing, which employs not only the amplitude of a resonantly reflected light wave, but its phase as well. In this connection, the phase behavior under SPR has been comprehensively described by theoretical analysis, numerical simulations, and a number of experiments. Near optimum SPR conditions, a resonant phase dependence is step-like, the ‘step’ being at the reflectivity minimum. For SPR-based sensors, the slope of the ‘step’ can always be made by several orders steeper than that of the resonant reflectivity contour. The ‘step’ has been imaged by the fringe of a 2-dimensional interference pattern where one coordinate was the incidence angle, and the other was the phase. The inversion of the ‘step’ has been observed for the first time during antigen–antibody binding, when the system passes through the optimum SPR conditions. Monitoring the inversion provides for ultra-high sensitivity to an analyte while recording angular position of the ‘step’, does for dynamic range as wide as that of traditional SPR sensors. The SPR interferometry technique has confirmed theoretical findings and opened up new possibilities for (bio)chemical sensing.

Surface plasmon resonance interferometry for micro-array biosensing

Interferometry that detects the phase of a beam reflected under surface plasmon resonance (SPR) has been developed for bio and chemical sensing. The conditions have been found, under which the phase reveals abrupt jumps in response to a minute increase in the effective thickness of a receptor layer that binds analyte particles on the sensor surface. This forms the basis for biosensing with sensitivity much higher as compared to traditional SPR sensors. Besides, SPR interferometry (SPRI) provides spatial resolution at the micron scale. The enhanced sensitivity attributed to the phase jump and interferometric imaging of variations of the phase over the surface are demonstrated, which open up new avenues for micro-array biosensing.

Biocomputing based on particle disassembly

Nanoparticles with biocomputing capabilities could potentially be used to create sophisticated robotic devices with a variety of biomedical applications, including intelligent sensors and theranostic agents. DNA/RNA-based computing techniques have already been developed that can offer a complete set of Boolean logic functions and have been used, for example, to analyse cells and deliver molecular payloads. However, the computing potential of particle-based systems remains relatively unexplored. Here, we show that almost any type of nanoparticle or microparticle can be transformed into autonomous biocomputing structures that are capable of implementing a functionally complete set of Boolean logic gates (YES, NOT, AND and OR) and binding to a target as result of a computation. The logic-gating functionality is incorporated into self-assembled particle/biomolecule interfaces (demonstrated here with proteins) and the logic gating is achieved through input-induced disassembly of the structures. To illustrate the capabilities of the approach, we show that the structures can be used for logic-gated cell targeting and advanced immunoassays.

Magnetic Immunoassay for Detection of Staphylococcal Toxins in Complex Media

Method of highly sensitive registration of magnetic nanoparticles by their nonlinear magnetization is used in a novel sandwich-type immunoassay for detection of staphylococcal toxins in complex media of virtually any volume, with increasing sensitivity at higher sample volume. The signal is read out from the entire volume of a nontransparent 3D fiber structure employed as a solid phase, which provides large reaction surface, quick reagent mixing, as well as antigen immunofiltration directly in the course of the assay. The method has demonstrated near-linear dose-response curves within a wide range of ~3 decades, while detection of staphylococcal enterotoxin A (SEA) and toxic shock syndrome toxin (TSST) in neat milk without sample preparation. The limits of detection (LOD) as low as 4 and 10 pg/mL for TSST and SEA, respectively, were obtained in 2-h format using 30-mL samples. The second, 25-min format, showed the LOD of 0.1 and 0.3 ng/mL for the same toxins in a 150 μL sample. The developed immunoassay can be applied in food safety control, in vitro diagnostics, and veterinary for a variety of research from express tests in the field to highly sensitive laboratory tests.

Surface plasmon resonance bio- and chemical sensors with phase-polarisation contrast

Abstract Phase-polarisation contrast (PPC) method for the enhancement of the contrast of Surface Plasmon Resonance (SPR) information pattern is proposed and realised. The method consists in the optimisation of the optical scheme for SPR dip visualisation, that makes use of the behaviours of light phase and polarisation under SPR. A converging light beam with a variable polarisation ellipticity is used for surface plasmon excitation, while the reflected beam is analysed by a phase compensator and an analyser. Using the PPC method, we have observed the enhancement of the SPR pattern contrast by more than one order of magnitude in comparison with traditional SPR schemes. This allowed us to increase significantly the accuracy of SPR shift measurements in bio- and chemical sensing and to reduce restrictions on allowable dispersion of parameters of SPR-supporting metal films and binding layers immobilised on them. In addition, the PPC method can provide a gain in sensitivity.

Phase-polarisation contrast for surface plasmon resonance biosensors.

A technique of phase-polarisation contrast (PPC) for the enhancement of the contrast of a surface plasmon resonance (SPR) intensity profile is proposed and experimentally realised. The technique exploits the peculiarities of light phase and polarisation behaviour under SPR. It applies to non-optimum SPR coupling conditions and enables one to lower the resonant minimum of reflected intensity nearly to zero, and hence to increase substantially the ratio of the intensity from the resonance to that at the minimum. We observed the contrast enhancement by more than one order of magnitude when we applied the PPC scheme. The PPC can be efficiently employed in commercial SPR sensors, as it significantly reduces restrictions on allowable parameters of SPR-supporting metal films and biomolecular layers immobilised on them, facilitates SPR observation, and increases the accuracy of SPR shift measurements.

Experimental study of spontaneous electric field generated by a laser plasma

We report investigations of a space-time structure of the electric field of laser plasma from a solid target. An ArF excimer laser with an intensity of I≅108 W/cm2 was used to produce the plasma on various targets placed in air at atmospheric pressure. A strong difference in both the amplitude (by more than 1–2 orders of magnitude) and the structure of the electric field for conductive and dielectric targets has been observed. The field distribution for a conductive target was found to correspond to a dipole configuration of charges in the laser plasma, while for the dielectric target a quadrupole configuration was revealed. Possible explanations and applications of the observed effect are discussed.

Quantitative real-time in vivo detection of magnetic nanoparticles by their nonlinear magnetization

A novel method of highly sensitive quantitative detection of magnetic nanoparticles (MP) in biological tissues and blood system has been realized and tested in real time in vivo experiments. The detection method is based on nonlinear magnetic properties of MP and the related device can record a very small relative variation of nonlinear magnetic susceptibility up to 10−8 at room temperature, providing sensitivity of several nanograms of MP in 0.1ml volume. Real-time quantitative in vivo measurements of dynamics of MP concentration in blood flow have been performed. A catheter that carried the blood flow of a rat passed through the measuring device. After an MP injection, the quantity of MP in the circulating blood was continuously recorded. The method has also been used to evaluate the MP distribution between rat’s organs. Its sensitivity was compared with detection of the radioactive MP based on isotope of Fe59. The comparison of magnetic and radioactive signals in the rat’s blood and organ samples demon...

Dark-field surface plasmon resonance microscopy

Dark-field microscopy in a surface plasmon resonance (SPR) scheme is introduced. A nanometre-thick patterned coating was imaged with SPR, and the image contrast was drastically enhanced when the zero diffraction order was eliminated. The resulting contrast is assigned to abrupt changes of the light phase with coating thickness, which are specific to SPR. The method offers prospects for highly sensitive multiple-spot biosensors.