Rapid and accurate detection of fentanyl (highly potent opioid) is a critical importance due to current opioids crisis worldwide. We report the highly sensitive detection of fentanyl utilizing the synergetic effect of nanoporous silicon as a substrate with a high interfacial area and specific antibody functionalization of nanoporous silicon. The electrochemical sensor consists of gold working electrode (WE) and counter electrode (CE) deposited on nanoprous silicon, antibodies immobilized between these electrodes, and an Ag/AgCl reference electrode (RE). Square wave voltammetry (SWV) was used as an electrochemical transduction method. The detection limit was determined as 6 and 11.5 ng/mL for specific peak (fentanyl signature) in phosphate buffer (PB) and human sweat, respectively. A future goal of this study is to develop a wearable sweat sensor array for rapid and on-site detection of multiple opioids with analytical sensitivity comparable with laboratory tests.

The direct conversion of light energy into to mechanical motion, the optomechanical effect, is also termed photostriction or photoactuation. Photostrictive materials are scarce but the technology is of significant interest with applications ranging from civil infrastructures to medicine to the aerospace industry to telecommunications. These types of smart materials are expected to be able to produce a variety of devices and are envisioned to be used as light-driven motors, for energy scavenging, artificial muscles, and tactile displays. The development goals for photoactuators include large range of motion, high precision, high speed, high strain energy density, and low fatigue rate. Ideally, these types of materials could be used in both microelectromechanical and nanoelectromechanical systems. Carbon nanotubes (CNTs) have been use to construct electrically driven actuators using a variety of structures [1–7]. Generally, in the electrically driven systems the charge separation at or near the electrodes effect the biggest component of the volume change. There has been a flurry of theoretical activity to understand the mechanical properties of CNTs [8–19]. However, none of these reports considered the effect of light on the stress/strain behavior of CNTs. CNTs can be observed to photoactuate under a variety of motifs, including freestanding bundles [20, 21], pure CNT films [22], mixed into polymer hosts [23–27], and as part of layered composites [28–33]. While the magnitude of the volume response under exposure to light is remarkably large in all cases, the exact composition and preparation method used to make the photoactuator appears to have an influence on the underlying mechanism. Chapter 2 Use of Carbon Nanotubes in Photoactuating Composites

Transparent conductive meshes were fabricated by inkjet printing on flexible substrate using a percolation pattern created by random removal of conducting bonds from a regular square two-dimensional lattice. With this approach, a higher gain in optical transmittance than electrical conductivity loss is achieved above the percolation threshold. As a result of this, a figure of merit for the percolation pattern is improved with respect to a regular square mesh. The transmittance (T), sheet resistance (R), and figure of merit (F) on percentage of removed bonds for square lattices were measured. The gain of the figure of merit was observed in the range of removed bonds from 5% to 15%. Our best samples exhibit T = 84%, R = 1.3 Omega/sq, and F = 130 X 10(-3) 52(-1) (highest F value and lowest R value) and T = 93%, R = 8 Omega/sq, and F = 65 X 10(-3)Omega(-1) (highest T value). This demonstrates an excellent transparent conductive film (TCF) performance and is significantly better than any continuous TCF. The percolation meshes demonstrate good mechanical stability and the absence of a Moire effect. A distinctive feature of this method is its universality and capability of being adapted to any symmetrical or asymmetrical pattern and deposition technique.

We report the highly sensitive and selective detection of hydrogen peroxide vapors via fluorescence enhancement (turn-on mechanism) for boronate quinoline chromophore deposited on a flat substrate and entrapped in a nanoporous silicon multi-layered structure. The detection limit was determined to be 150 ppt with a negligible response to major volatile organic compounds and water. A distinctive feature is extraordinary sensitivity which critically depends on amount of the sensory material infiltrated inside nanoporous silicon.

We report a hybrid solar cell based on single walled carbon nanotubes (SWNTs) interfaced with amorphous silicon (a-Si). The high quality carbon nanotube network was dry transferred onto intrinsic a-Si forming Schottky junction for metallic SWNT bundles and heterojunctions for semiconducting SWNT bundles. The nanotube chemical doping and a-Si surface treatment minimized the hysteresis effect in current–voltage characteristics allowing an increase in the conversion efficiency to 1.5% under an air mass 1.5 solar spectrum simulator. We demonstrated that the thin SWNT film is able to replace a simultaneously p-doped a-Si layer and transparent conductive electrode in conventional amorphous silicon thin film photovoltaics.

We present a short review of recent progress in the field of optical gas sensors based on porous silicon (PSi) and PSi composites, which are separate from PSi optochemical and biological sensors for a liquid medium. Different periodical and nonperiodical PSi photonic structures (bares, modified by functional groups or infiltrated with sensory polymers) are described for gas sensing with an emphasis on the device specificity, sensitivity and stability to the environment. Special attention is paid to multiparametric sensing and sensor array platforms as effective trends for the improvement of analyte classification and quantification. Mechanisms of gas physical and chemical sorption inside PSi mesopores and pores of PSi functional composites are discussed.

Herein we report the highly efficient and sensitive detection of hydrogen peroxide in both aqueous solution and in the vapor phase via fluorescence quenching (turn-off mechanism) of the amplified fluorescent conjugated polymer-titanium complex induced by hydrogen peroxide. Inter- and intra-polymer energy migration leads to extremely high sensitivity.

Schottky diodes and solar cells are statistically created in the contact area between two macroscopic films of single-walled carbon nanotubes (SWNTs) at the junction of semiconducting and quasi-metallic bundles consisting of several high quality tubes. The n-doping of one of the films allows for photovoltaic action, owing to an increase in the built-in potential at the bundle-to-bundle interface. Statistical analysis demonstrates that the Schottky barrier device contributes significantly to the I - V characteristics, compared to the p-n diode. The upper limit of photovoltaic conversion efficiency has been estimated at ∼20%, demonstrating that the light energy conversion is very efficient for such a unique solar cell. While there have been multiple studies on rectifying SWNT diodes in the nanoscale environment, this is the first report of a macroscopic all-carbon nanotube diode and solar cell.

We investigate the exciton energy transfer (ET) in nanoassemblies (nanotube based aggregates) formed by polymer wrapped single-walled carbon nanotubes (SWNTs) using photoluminescence (PL) spectroscopy and simulation. The distinctive feature of this study is the gradual growth of such nanostructures in aqueous medium induced by increasing the concentration of porphyrin molecules stitching nanotube-polymer complexes in densely packed assemblies. Experimental dependencies of PL intensity on the porphyrin concentration for different types of semiconducting SWNTs demonstrate step-like behavior controlled by the amount of bound nanotubes and are in good agreement with the simulating model. The simulation algorithm determines the criterion of the aggregate formation depending on the number of porphyrin molecules per tube and the cascade exciton energy transfer between neighboring semiconducting nanotubes of different chiralities. Aggregates of small sizes (up to six-eight individual tubes) contain mostly semiconducting species, while aggregates of a larger size (up to several tens of tubes) incorporate metallic SWNTs, inducing strong PL quenching. From the fitting procedure, an ET rate of 0.6 × 10(10) s(-1) has been determined which is consistent with the center to center distance (∼2.3 nm) between adjacent tubes separated by polymer and porphyrin molecules. The threshold of the dimer formation corresponds to one porphyrin molecule per ∼20 nm of tube lengths that was supported by molecular dynamics simulation. These findings provide insight into the ET mechanism in SWNT nanoassemblies of variable sizes, which can be gradually controlled by the external factor (the concentration of porphyrin molecules).

We present the study of a nanohybrid composite with superior sensing performance consisting of an emissive sensory polymer infiltrated into a mesoporous Si one-dimensional (1D) photonic crystal with a microcavity (MC). It was found that the critical condition for deep polymer infiltration is the presence of an initial low porosity layer (porosity of 45%) in contrast to shallow infiltration governed by an initial high porosity layer (porosity of 58%). This results in a narrow fluorescence peak (due to deep infiltration) or a spectral "hole" in the fluorescence band (shallow infiltration). Such a unique effect is in agreement with the model based on capillary filling and confirmed by secondary ion mass spectrometry (SIMS) data analyzing the profile of polymer infiltration along the MC depth. In the case of deep infiltration, the characteristic filling length exceeds 2 pm, allowing the polymer to impregnate the MC layer. The infiltrated polymer is spatially confined and exists as quasi- isolated chains without pore clogging as can be concluded from the "blue" spectral shift of up to 10 nm as compared with a nonspatially confined film. Polymer isolation over a large surface area along with sufficient pore openings makes this porous Si (PSO MC/polymer nanohybrid an ideal material for gas sensing applications. This is due to the high sensitivity in conjunction with a strong fluorescence signal which is not possible with solid polymer films or bare PSi. These results are confirmed by direct observation of higher sensitivity, enhanced specificity, and partial recovery of the optical signal for the nanohybrid composite upon exposure to trinitrotoluene vapors as compared with a conventional polymer film deposited on a flat substrate.

This short review describes recent results in the field of carbon nanotube (CNT) – Si hybrid photovoltaics (PV) focusing on advantages of semiconducting carbon nanotubes over other organic materials used in organic- Si composite photosensing materials. Possible mechanisms of charge phogeneration at CNT- Si interface and chargte transport are discussed. Perspectives and future trends in research of this novel class of PV nanohybrids are presented as well.

This editorial presents a short summary and perspectives of recent explorations of light energy conversion and related photovoltaic (PV) devices constructed from carbon nanotubes (CNTs) interfaced with organic and inorganic materials. The first reports about CNT photoconductivity [1-3] motivated a growing interest of light energy conversion employing unique optical, electrical, thermal and mechanical properties of carbon nanotubes. Carbon nanotubes as organic, nanoscaled objects [4] outperform their organic counterparts in many aspects, making CNTs the favorite candidate for various optoelectronic applications. For example, semiconducting CNTs have a high light absorbance in visible and near infra-red (NIR) spectrum (absorbance coefficient is in the range of 10 4 –10 5 cm −1 ), with a band gap depending on their diameter, while most organic compounds are not capable of absorbing NIR light. Another advantage of CNTs is very high charge mobility (up to 10 5 cm 2 V −1 s −1 for individual nanotubes [5] and ~60 cm 2 V −1 s −1 for CNT films [6]) as compared with conductive organic materials. In addition, carbon nanotubes exhibit an exceptional environmental stability and resistance to photo bleaching which is one of the major drawbacks of organic optoelectronics. Device fabrication is simple and cost effective as CNTs can be easily incorporated in the device’s architecture by wet processing (coating, spraying, and printing). Finally, CNTs can be simply doped or functionalized by many covalent and non-covalent routes, forming nano-assemblies with other molecules and polymers to provide an efficient photoinduced charge transfer or tuning Fermi level to the favorable position at heterojunction with semiconductors. CNT light absorption is an initial step leading to a generation of bound excitons. In order to convert light energy into an electrical signal, excitons should be separated on free charge carriers (electrons and holes) by an external or internal built-in electric field, before they relax to the ground state. Finally, the resulting carriers should be transported to the electrodes minimizing the recombination and trapping processes. Such a scenario is realized for photovoltaics and photodetectors when the internal built-in field is required at the interface between carbon nanotubes and other materials. Distinct from pristine CNTs, interface related photoconversion processes are more complex but at the same time very intriguing, rich in novel phenomena and are extremely attractive for many optoelectronics applications. For instance, very recently, a surprisingly high photoconversion efficiency (PCE) of ~14% for CNT/Si hybrid solar cells has been reported [7], exceeding any PCE for organic and hybrid photovoltaics. Noteworthy, research in the field of CNT/semiconductor PV is very new (just past five years) and limited by few groups [7-10] as compared with substantial efforts and time (about 20 years) spent by PV community in other directions such as polymer based and dye-synthesized solar cells where the best PCE is still in the range of 10-12% [11-13]. Photoactive CNT/ counterpart nanohybrids include significant diversity in the CNT morphology (network and individual CNTs), their structure (SWNTs and MWNTs) and electronic properties (semiconducting and metallic). The choice of CNT counterpart can also be very different comprising small molecules, oligomers, polymers, quantum dots and semiconductors (bulk and nanostructured). The most studied structures utilized in solar cell design are CNTs/small molecules and CNTs/polymers, where CNTs act as an electron acceptor (with some exceptions) and light is absorbed through the CNT complimentary component. Interestingly, in majority PV studies of CNT hybrids, the role of CNTs in light harvesting was underestimated in the photocorversion process. Recent reports demonstrated that CNTs can be involved not only in charge separation and transport processes, but also in efficient light absorption [9-14]. It was a great hope in the beginning of 2000 th that an incorporation of carbon nanotubes in organic photovoltaic device (OPV) and formation of bulk heterojunction between CNT and polymer or small molecules should significantly improve solar cell performance and boost PCE level beyond of 10%. Nevertheless, despite substantial efforts for almost a decade, the maximum reported PCEs for CNT/polymer devices stay in the range of only 0.2-0.5% [15,16]. Just recent studies [17,18] shed light on exciton dissociation, role of semiconducting and metallic carbon nanotubes and the morphology of single walled carbon nanotube (SWNT) networks. It was demonstrated that the critical role in efficient charge separation and transport in SWNT/polymer heterojunction play semiconducting SWNTs (not metallic) and their debundling minimizing the junction effect. As a rule, the inter-tube junctions in SWNT network is the source of energy barriers and trapping, that substantially reduces the transport of photogenerated carriers. Alternatively, to CNT based OPVs, CNT/semiconductor hybrids opened recently a new horizons in light harvesting for renewable energy sources. Exciting results ( PCE ~14% ) for CNT/Si PV heterostructure reported last year [7] promise novel achievements in the near future toward stable conversion efficiency exceeding 15%. Especially, CNT can be effectively employed in amorphous Si (aSi) thin film, flexible solar cells, replacing p-type aSi layer. The cost of carbon nanotubes reduces very rapidly every year, while the conversion efficiency of 15% and higher definitely exceeds any organic/hybrid/aSi (~8-12%) thin film devices in the current market. Last but not least, there is an exceptional environmental stability of carbon nanotubes, significantly outperforming any organic compounds. Moreover, the variety of parameters (e.g. CNT chirality, diameter, doping level, Si porosity, etc) allow to further proceed with device optimization leading to 18-20% PCE level. Cost-effective processing, light weight, flexibility and robustness make these PV devices extremely attractive for various civilian needs using clean technology approach. It is hard to overestimate the benefits of the emergence of a novel and highly competitive CNT nanocomposite based solar cells in the current multibillion dollar market.

An anisotropic carbon nanotube (CNT)‐polymer composite for bolometric applications in the mid‐IR spectral range (2.5–20 μm) is studied. Composite alignment in conjunction with non‐uniform distribution of CNTs in the polymer matrix allows for a significant enhancement of the temperature coefficient of resistance (0.82% K−1) with respect to uniform composite (0.24% K−1). As a result a responsivity of ≈ 500 V W−1 is reached, which is the highest for CNT‐based bolometers reported to date. Such remarkable optical and thermal characteristics are explained in terms of fluctuation tunneling theory taking into account the composite anisotropy and the gradient of the CNT concentration. Flatness of the photoresponse in the broad spectral mid‐IR range and enhanced responsivity provide a great potential for the use of such novel composite for applications in IR spectroscopy and thermal imaging.

SummaryUnusual aggregation of biopolymer‐wrapped single‐walled carbon nanotubes (SWCNTs) induced by cationic porphyrin meso‐tetrakis(4‐N‐methyl‐pyridyl)porphine (TMPyP4) in aqueous suspension has been studied by absorption spectroscopy and molecular dynamics simulation. After the addition of a small dose of TMPyP4 (10−6 M) into poly(rC):SWCNTs aqueous suspension, nanotubes begin to aggregate, but the aggregates are stable without precipitation during a few days. Spectral observations indicate that the porphyrin core stacked with π‐systems of the biopolymer:SWCNT hybrid and all nanotube species take part in this aggregate formation without any selection. Molecular dynamic simulation demonstrates that the stable complex consisting of two nanotubes can be formed when TMPyP4 molecules bound nanotubes both owing to π‐π stacking of the porphyrin core with the surface of one nanotube and with the oligonucleotide adsorbed onto another nanotube. TMPyP4 molecules not only couple the biopolymer:nanotube hybrids, but also they stitch polymer's strands wrapping around each nanotube resulting in continuous floatation of aggregates in water. It was shown that the aggregate is stable even at 373 K. At this temperature additional contacts between two neighbor's polymers appear which strengthen this aggregate too. The controlled aggregation of biopolymer‐wrapped carbon nanotubes can be employed at the fabrication of multifunctional supramolecular assemblies, photovoltaic cells and in biosensing.

We demonstrate that the electronic structure of mesoporous silicon is affected by adsorption of nitro-based explosive molecules in a compound-selective manner. This selective response is demonstrated by probing the adsorption of two nitro-based molecular explosives (trinitrotoluene and cyclotrimethylenetrinitramine) and a nonexplosive nitro-based aromatic molecule (nitrotoluene) on mesoporous silicon using soft X-ray spectroscopy. The Si atoms strongly interact with adsorbed molecules to form Si-O and Si-N bonds, as evident from the large shifts in emission energy present in the Si L(2,3) X-ray emission spectroscopy (XES) measurements. Furthermore, we find that the energy gap (band gap) of mesoporous silicon changes depending on the adsorbant, as estimated from the Si L(2,3) XES and 2p X-ray absorption spectroscopy (XAS) measurements. Our ab initio molecular dynamics calculations of model compounds suggest that these changes are due to spontaneous breaking of the nitro groups upon contacting surface Si atoms. This compound-selective change in electronic structure may provide a powerful tool for the detection and identification of trace quantities of airborne explosive molecules.