Sensors Based on SPR and LSPR Phenomena

Surface plasmon resonance is exceptionally sensitive to changes in the refractive index of the medium near the surface of the metal film or nanoparticle due to the nanosized localization and amplification of the EM field. The measurements of angular SPR spectra or LSPR spectra in the light extinction or scattering modes register changes in the local refractive index due to the presence of molecules. In particular, the SPR angular position or the LSPR peak wavelength can be correlated with molecular adsorption, desorption, or conformational changes that cause changes in the refractive index. The high sensitivity of the nanoparticle EM near field in combination with the advantages of light (non-destructive action, high speed, and direction) makes the SPR and LSPR (plasmonic) sensors promising for the study of biological molecules and reactions.

An important stage for the creation of functional SPR sensors is the development of physical criteria for determining the complex refractive index and geometric parameters of the layered molecular structures and recording changes in these parameters. It was shown in Ref. [4] that the processes of surface plasmon polariton resonance excitation in the Kretschmann configuration sensors are well described by a one-dimensional model of a multilayer system based on the effective optical constants matrix and layer thicknesses, which takes into account the presence of transition layers and the imperfectness of the geometric surface. Due to this, an approach was developed that allows the use of the SPR angular spectrum shape to determine by mathematical analysis at least three parameters (effective refractive index and absorption coefficient, thickness) of the investigated structure.

An important area of SPR research is the development of new sensing techniques, in particular, to expand the capabilities and improve the limit of detection of sensor devices. For example, Lavine et al. [5] deposited molecularly imprinted lightly crosslinked N-[N- propyl)acrylamide granules on a glass plate covered with a thin gold layer (SPR chip) by the preparation of thin organic layers by centrifugation (spincoating), and the swelling of these granules was used to measure the concentration of theophylline. Gabai et al. [6] performed measurements of glucose concentration using copolymer films of "boronic acid/acrylamide" on the SPR chip surface made by electropolymerization.

It is known that the SPR chip angular reflectivity spectrum significantly depends on the electromagnetic interaction between the surface plasmon polariton that is excited on the metal surface and the localized surface plasmon of the metal nanoparticles located on this surface, which is expressed in the minimum angle shift of the SPR curve and the increase in reflection [7, 8]. It has been shown that this physical effect can be used to enhance the molecular biorecognition events [9-11] and biocatalytic transformations [12,

13] with the use of gold or silver nanoparticles as labels.

For the creation of novel sensor devices, electrochemical SPR (ESPR) spectroscopy is a promising method, which is based on the combination of the SPR measurements with the electrochemical control of molecular processes occurring on the metal-electrolyte boundary. In particular, the ESPR method was used to detect cofactor molecules [14], glucose [6, 15], and hydrogen peroxide [16]. The applicability of the method for the investigation of the influence of electric field on the molecular systems properties has been shown.

In Ref. [17], the Stark effect in molecular adsorbates at different light wavelengths was studied using SPR spectroscopy. The influence of the applied electrostatic field on the processes of hybridization and DNA denaturation was investigated with SPR in Ref. [18]. By measuring reflectivity, the dependence of the degree of soybean peroxidase immobilization on the gold substrate on the magnitude of the applied electric field [19] was recorded.

As in the case of SPR, at the initial stage of the LSPR sensor development, it is necessary to use mathematical modeling in order to evaluate the potential sensitivity of the sensor and optimize its parameters. Lee and El-Sayed [20] studied the metallic nanorods LSPR spectrum sensitivity dependence on changes in the refractive index of environment, which depend on the size, shape, and metal type of nanorods. Yan et al. [21] have shown that the position of the LSPR absorption peak, its half-width, and the intensity increase nonlinearly with the increase in the shell thickness of the "gold core-dielectric shell” nanostructures. Xu and Kail [22] developed theoretical approaches to take into account the particle-particle and substrate-particle interactions in the model of LSPR sensors based on gold nanoparticles. Westcott et al. [23] studied the spectral properties of LSPR extinction of the "dielectric core-metal shell” nanostructures. Haes et al. [24] showed that an increase in the ratio of the silver nanostructure geometric sizes with the shape of a cut tetrahedron provides greater shifts in the LSPR peak in the light extinction spectrum when the dielectric coating is formed on its surface. Murray et al. [25] found that gold nanorods provide greater sensitivity to changes in the local refractive index of the environment than nanoparticles with disk shapes. Malinsky et al.

[26] theoretically studied the sensitivity of the silver nanoparticles LSPR extinction peak position to the changes in the refractive index of the environment in the model based on the Mie theory.

The LSPR properties of multilayer nanoparticles of different geometry and composition are actively studied, and these results can be used to create highly sensitive LSPR sensors. Khlebtsov et al. demonstrated that the sensitivity to a biomolecular coating of quartz nanospheres coated with a layer of gold may be higher compared to the sensitivity of spherical gold nano particles with the same volume

[27] . In Refs. [28, 29], the same authors proposed a multilayer model for gold and silver nanoparticles, which allows describing the interaction between biomolecules immobilized on nanoparticles and analyte molecules in a solution. Wu et al. [30] showed significant sensitivity of three-component nanostructures of Si02-Ag-Au and Si02-Au-Ag types to the dielectric properties of the environment.

The practical implementation of sensors based on the LSPR phenomenon was preceded by experimental studies of the optical properties of metallic nanostructures. In 1995, Kreibig and Vollmer [31] demonstrated that the optical density of the immobilized monolayer of gold colloidal nanoparticles depends on the refractive index of the surrounding liquid medium. Several experimental works were published, which considered the influence of the parameters (for example, shape, size, and interparticle distance) of nanostructured systems on the properties of light extinction and optical dichroism [32-36]. Schatz et al. [37] and Van Duyne et al. [38,

39] showed that ordered monolayers of silver or gold nanostructures on the surface of mica or glass can be fabricated using nanosphere lithography, which allows registering the biomolecular interaction. Chumanov et al. [40] showed the possibility of creating stable monolayers of silver nanoparticles obtained from colloidal solutions on solid or flexible substrates by virtue of a transition polymer layer. Such arrays of silver nanostructures exhibit extremely narrow peaks in the extinction spectra, which may be promising for the creation of highly sensitive biosensors.

Yonzon et al. studied the binding of Concanavalin A to mannose- functionalized nanoparticles in real time [41]. Haes et al. used LSPR sensor to detect ligands (amyloid derivatives capable of diffusion into biological tissues) at a concentration of 100 fM [42]. Alivisatos et al. developed a kind of plasmon molecular ruler, which measures the modulation of the LSPR spectrum depending on changes in the electromagnetic interaction caused by changing the distance between a pair of metal nanoparticles, to detect the hybridization of DNA oligonucleotides to single-stranded DNA [43]. Recently, 3D plasmonic molecular rulers have been developed based on bundled nanoparticles (plasmonic oligomers); 3D rulers allow getting the full spatial configuration of biological processes and their dynamic development. Atwater et al. developed elastic plasmonic materials [44]. The integration of split ring resonators into polydimethylsiloxane allowed, through the mechanical deformation of the polymer, changing the strength of the electromagnetic interaction between the resonators, which makes it possible to regulate the response of the metamaterial.

Since molecular resonances lead to spectrally selective optical absorption by molecules and using an electronic coupling between their molecular resonance and nanoparticles LSPR, mechanisms of significant changes in the LSPR spectra were discovered upon overlapping of the aforementioned resonances [45]. Such increased sensitivity to molecular absorption opens a way for the creation of highly sensitive resonance biosensors. Wiederrecht et al. reported hybridization in J-aggregate-metallic nanosphere complexes [46]. Halas et al. studied wavelength-dependent behavior of hybrid nanostructures formed by Au nanoshells and J-aggregates [47]. Au nanoshells made it possible to easily adjust the LSPR wavelength in a wide spectral range around the absorption peak of the J-aggregates. Wang et al. used Au nanorods to study the resonance interaction with H-aggregates at different LSPR wavelength positions of nanorods [48]. To solve the issues related to the fixed LSPR position in metallic nanoparticles, Zheng et al. have developed tunable plasmonic systems with variable light incidence angle changing the LSPR spectrum [49].

The LSPR phenomenon in high-conductive nanostructures can also be used to localize and amplify the electromagnetic field in order to provide conditions for increasing the efficiency of optical transitions in molecular systems. For example, if a fluorophore molecule is placed near the nanostructured surface of a high- conductive metal, then under certain conditions one can observe an increase in the intensity of its emission compared with the case of the absence of nanostructures [50-52]. Sensors constructed on this principle give an opportunity to increase the sensitivity of fluorescence measurements (for example, to capture a signal even from individual molecules [53, 54]], which determines their potential for applications in the biochemistry and medicine fields. However, enhancement of the dyes’ fluorescence on silver and gold nanostructures substantially depends on the conditions of the resonant energy transfer of plasmon oscillations from the nanostructured metal surface to the dye molecule located near this surface [55-57]. Therefore, studies were actively carried out on the fluorescence enhancement affected by the shape and size of the metallic nanostructures themselves [58], the distance between the fluorophore molecules and the plasmon-generating surface [50, 55, 59], and also the characteristics of the molecule, such as its quantum yield and excitation lifetime [55, 56, 60]. Sorokin et al. showed that the fluorescence in the J-aggregates of cyanine dyes was enhanced when colloidal silver nanoparticles (two times) and gold nanostructure arrays (eight times) coated with polyelectrolyte layers were used to amplify the signal [61, 62]. In both cases, the optimal total thickness of the polymer layers was 16 nm. The simulation of the emission of fluorescent molecule layer with a thickness of 5 nm on a spherical gold nanoparticle with a diameter of 80 nm showed that the optimal distance between the fluorophore and the metal surface when the emission enhancement is observed is about 20 nm; in other cases, smaller amplification or quenching was observed [60]. In Ref. [63], in an experimental study of dyes emission on silver and gold nanostructures, it was noted that for monitoring fluorescence amplification, the distance between the dye and the metal surface should be 24-25 nm, and the fluorescence quenching is observed at a distance of 15 nm. Theoretical calculations [60] showed that the greatest increase in fluorescence is possible when the dipole moments of the molecules are directed normally to the plane of the nanostructured surface and at an optimal distance between the molecule and the metal surface. The magnitude of the molecular fluorescence intensity enhancement that can be achieved is from several to tens of times, according to literature data [53, 64].

To summarize, improvement in sensitivity and selectivity of plasmonic sensors is achieved through progress in a number of aspects: modeling and manufacturing of metallic nanostructures; surface functionalization; understanding of the interactions between the surface plasmon and the molecule.

Research in Material Science Field

One of the promising applications of molecular plasmonics is the non-destructive study of the properties of thin organic and inorganic films. Thus, the ESPR method was used for the study of electropolymerization processes [65] and the study of redox properties of polymers [66]. Damos et al. [67] performed electropolymerization of ultrathin films of methylene blue and investigated them using an ESPR method in real time. In Ref. [14], ESPR method was demonstrated for photonic transduction of the redox properties of an inorganic three-dimensional polymer Prussian blue. For the three redox states of this substance, different spectra of the SPR were observed; considering that the redox state does not affect the thickness of the film, these differences were explained by the change in the refractive index of the polymer. Thus, in this experiment, the electrochemical information, which is typical for the three redox states, has been converted into optical information by means of SPR, indicating the perspective of memory devices development for photonics with three stable states. Reference [14] demonstrates the fundamental possibility of using complex multi-stage redox transformations to create stable and reproducible photonic systems with multiple switching.

Due to its high sensitivity and surface nature, SPR method makes it easy to excite molecules and track their relaxation [68]. Therefore, it is ideally suited for studying conformational changes in gel-like polymeric materials by registering changes in the refractive index during the measurement of the object being studied in real time. In Ref. [6], the immobilization of an acrylamidophenylboronic acid- acrylamide copolymer by an ESPR is considered with the subsequent study of cyclic glucose-induced swelling of a polymer. The structure of the hydrogel (thickness, liquid saturation) and the kinetics of glucose-induced swelling and shrinking were studied. Investigation of these processes in acrylamidophenylboronic acid-acrylamide copolymers opens prospects for their use as matrices in sensors for glucose or in glucose-activated drug release systems.

An important application of SPR-based methods is the study of various properties of nanomaterials and nanostructured systems. In Ref. [69], it was found that charging a gold nanoparticle by electrons or removing electrons from it leads to significant shifts in the LSPR band. These spectral shifts were explained by changes in plasma frequency caused by the growth of charge density, which is the result of the metal nanoparticles electrolytic charging [70]. For example, the transformation of gold nanoparticles to the electron-depleted state by changing the voltage from -0.16 V to 0.82 V (relative to the silver quasi-reference electrode) induces a shift in the LSPR spectrum toward lower energies [69]. In the study in Ref. [71], with the help of SPR spectroscopy and electrochemical measurements, photoelectrochemical charging of gold nanoparticles attached to the gold surface through the auxiliary monolayer of cystamine was demonstrated by using the light excitation of CdS quantum dots bound to gold nanoparticles.

It is worth noting that SPR phenomenon and methods on its basis play a significant role in the creation and research of nanostructured materials with unique optical properties, so-called metamaterials, which in recent years have been actively studied due to promising applications in the fields of laser optics, optoelectronics, and chemical and biological sensors [72-75]. An important contribution to the production of such metamaterials can be the use ofplasmonic nanostructures, such as gold and silver nanoparticles, nanorods, nanodisks, nanorectangles, and nanoprisms [76-80]. Of particular interest are metamaterials, which the reversible variation of physical properties is possible in, for example, materials that exhibit a dynamic change in the LSPR parameters [81-83].

Promising Research Directions

Plasmonic nanoscopy and visualization

Light has significant advantages for visualization, including remote and non-destructive character and short response time. However, the complexity of optical imaging of nanoscale objects is associated with diffraction limitation. Advances in nanotechnology have helped to develop techniques that have overcome this limitation. For example, near-field scanning optical microscopy (plasmonic nanoscopy) allowed nano-dimensional biological and medical imaging with high spatial and time resolution. Estrada and Gratton [84] used the high resolution, typical for plasmonic nanoscopy, to study the fluorescence lifetime of a single molecule depending on the distance to the laser- illuminated gold nanoparticle, which also serves as a nanoscopic probe. This study helped to highlight the physical mechanisms associated with quenching and enhancement of the dye fluorescence during LSPR excitation in nanoparticles. Using plasmonic nanoscopy, Estrada and Gratton obtained 3D images of biological fibers such as collagen and actin filaments with high resolution, moving a separate Au nanoparticle along the fibers in the near-infrared femtosecond pulses exposure mode and measuring its trajectory. Thus, metal nanoparticles with a special surface functionalization can serve as probes for 3D in vivo molecular imaging. "Chemical vision,” which combines nanoscale imaging and molecular recognition, is one of the most important new directions in plasmonic nanoscopy.

Applications based on thermal effects

With optical near-field effects, metal nanoparticles can quickly convert the energy of the absorbed photon into heat through optical absorption as a result of LSPR. Among the various metallic nanoparticles proposed for photothermal therapy, gold nanoshells, nanorods, and nanocontainers have been most studied due to their wide LSPR range that extends into the near-infrared spectral region, where absorption by living tissues is minimal. Elliott etal. determined the quantitative characteristics of the nanoshells' interaction with laser radiation to study the influence of the nanoshells' concentration and the laser power on the photothermal effect [85]. Stern et al. evaluated the effect of nanoshell concentration in mice on the treatment of prostate cancer [86]. When the authors directed the infrared laser light through the skin of mice to the tumor, resonance absorption of energy by nanoshells raised the local temperature of malignant formations from 37°C to 45°C, killing cancer cells and leaving the surrounding healthy tissue intact. El-Sayed et al. used gold nanoparticles covered with antibodies for targeted delivery and photothermal therapy of epithelial carcinoma [87].

For therapies that require chemical drugs or genes, rather than direct heat treatment, plasmon-enhanced photothermal effects can also be used to develop nanocarriers that will enable optically controlled delivery of drug/oligonucleotide molecules. Particularly, Huang et al. developed aptamer/DNA-gold nanoparticle nanocarrier for directed drug delivery [88]. When irradiating the nanostructure by light with a wavelength corresponding to LSPR, the shell may become heated to a temperature that destabilizes the bond between the nanoparticle and the molecule and leads to the release of the drug that allows the treatment of cancer cells in defined spatiotemporal intervals.

Mechanical applications

The ability to capture, retain, and control molecules or biomolecules with nanosized precision is important for the analysis and understanding of biochemical processes. Among the works in this direction, it is necessary to highlight studies on the development and use of plasmonic tweezers. In this approach, a laser beam focused on a plasmonic nanoparticle is used to hold and control the biomolecules in the near-field gradient zone of the nanoparticle at low laser radiation intensities. Miao and Lin have demonstrated that the LSPR near field enhanced by an array of self-assembled gold nanoparticles can be used to hold objects up to micron size at low laser intensities [89]. Electromagnetic interactions between adjacent plasmonic nanostructures can be used to achieve better control of the EM fields and to increase the plasmonic tweezers productivity. For example, in a closely spaced dimer of plasmonic nanoparticles, stable holding of small particles and molecules was observed. Grigorenko et al. reported the retention of molecules with a pair of gold nanoparticles in the standard configuration of optical tweezers [3].

 
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