Enhanced photochemical activity and ultrafast photocarrier dynamics in sustainable synthetic melanin nanoparticle-based donor-acceptor inkjet-printed molecular junctions.

Melanin is a stable, widely light-absorbing, photoactive, and biocompatible material viable for energy conversion, photocatalysis, and bioelectronic applications. To achieve multifunctional nanostructures, we synthesized melanin nanoparticles of uniform size and controlled chemical composition (dopamelanin and eumelanin) and used them with titanium dioxide to fabricate donor-acceptor bilayers. Their size enhances the surface-to-volume ratio important for any surface-mediated functionality, such as photocatalysis, sensing, and drug loading and release, while controlling their chemical composition enables to control the film's functionality and reproducibility. Inkjet printing uniquely allowed us to control the deposited amount of materials with minimum ink waste suitable for reproducible materials deposition. We studied the photochemical characteristics of the donor-acceptor melanin-TiO2 nanostructured films via photocatalytic degradation of methylene blue dye under selective UV-NIR and Vis-NIR irradiation conditions. Under both irradiation conditions, they exhibited photocatalytic characteristics superior to pure melanin and, under UV-NIR irradiation, superior to TiO2 alone; TiO2 is photoactive only under UV irradiation. The enhanced photocatalytic characteristics of the melanin-TiO2 nanostructured bilayer films, particularly when excited by visible light, point to charge separation at the melanin-TiO2 interface as a possible mechanism. We performed ultrafast laser spectroscopy to investigate the photochemical characteristics of pure melanin and the melanin-TiO2 constructs and found that their time-resolved photoexcited spectral patterns differ. We performed singular value decomposition analysis to quantitatively deconvolute and compare the dynamics of photochemical processes for melanin and melanin-TiO2 heterostructures. This observation supports electronic interactions, namely, interfacial charge separation at the melanin and TiO2 interface. The excited-state relaxation in melanin-TiO2 increases markedly from 5 ps to 400 ps. The results are remarkable for the future intriguing application of melanin-based constructs for bioelectronics and energy conversion.

Inkjet Printing of Synthesized Melanin Nanoparticles as a Biocompatible Matrix for Pharmacologic Agents.

Melanin is a natural biopigment that is produced by melanocytes and can be found in most living organisms. The unique physical and chemical properties of melanin render it potentially useful for numerous applications, particularly those in which a biocompatible functional material is required. Herein, we introduce one important technology in which melanin can be utilized: a drug delivery system in terms of a biocompatible matrix. However, extracting melanin from different biological sources is costly and time-consuming and introduces variabilities in terms of chemical structure, properties, and functions. Hence, a functionally reproducible system is hard to achieve using biologically extracted melanin. Here we report the synthesis of melanin nanoparticles of controlled uniform sizes and chemical characteristics. The optical, chemical, and structural characteristics of synthesized nanoparticles were characterized by optical confocal photoluminescence (PL) imaging, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Zeta potentiometry. The melanin nanoparticles have 100 nm size and a narrow size distribution. The advantage of a nanoparticle structure is its enhanced surface-to-volume ratio compared to bulk pigments, which is important for applications in which controlling the microscopic surface area is essential. Using the inkjet printing technique, we developed melanin thin films with minimum ink waste and loaded them with methylene blue (our representative drug) to test the drug-loading ability of the melanin nanoparticles. Inkjet printing allowed us to create smooth uniform films with precise deposition and minimum ink-waste. The spectroscopic analysis confirmed the attachment of the "drug" onto the melanin nanoparticles as a matrix. Hence, our data identify melanin as a material system to integrate into drug release applications.

Energy Band Gap Investigation of Biomaterials: A Comprehensive Material Approach for Biocompatibility of Medical Electronic Devices.

Over the past ten years, tissue engineering has witnessed significant technological and scientific advancements. Progress in both stem cell science and additive manufacturing have established new horizons in research and are poised to bring improvements in healthcare closer to reality. However, more sophisticated indications such as the scale-up fabrication of biological structures (e.g., human tissues and organs) still require standardization. To that end, biocompatible electronics may be helpful in the biofabrication process. Here, we report the results of our systematic exploration to seek biocompatible/degradable functional electronic materials that could be used for electronic device fabrications. We investigated the electronic properties of various biomaterials in terms of energy diagrams, and the energy band gaps of such materials were obtained using optical absorption spectroscopy. The main component of an electronic device is manufactured with semiconductor materials (i.e., Eg between 1 to 2.5 eV). Most biomaterials showed an optical absorption edge greater than 2.5 eV. For example, fibrinogen, glycerol, and gelatin showed values of 3.54, 3.02, and 3.0 eV, respectively. Meanwhile, a few materials used in the tissue engineering field were found to be semiconductors, such as the phenol red in cell culture media (1.96 eV energy band gap). The data from this research may be used to fabricate biocompatible/degradable electronic devices for medical applications.

Nanosensors for therapeutic drug monitoring: implications for transplantation.

The number of patients requiring organ transplantations is exponentially increasing. New organs are either provided by healthy or deceased donors, or are grown in laboratories by tissue engineers. Post-surgical follow-up is vital for preventing any complications that can cause organ rejection. Physiological monitoring of a patient who receives newly transplanted organs is crucial. Many efforts are being made to enhance follow-up technologies for monitoring organ recipients, and point-of-care devices are beginning to emerge. Here, we describe the role of biosensors and nanosensors in improving organ transplantation efficiency, managing post-surgical follow-up and reducing overall costs. We provide an overview of the state-of-the-art biosensing technologies and offer some perspectives related to their further development.

Grain-Resolved Ultrafast Photophysics in Cu2BaSnS4- xSe x Semiconductors Using Pump-Probe Diffuse Reflectance Spectroscopy and Microscopy.

In this paper, we analyze fundamental photoexcitation processes and charge carrier kinetics in Cu2BaSnS4- xSe x (CBTSSe), a recently introduced alternative to Cu(In,Ga)(S,Se)2 and Cu2ZnSnS4- xSe x (CZTSSe) photovoltaic/photoelectrochemical absorbers, using advanced laser spectroscopy and microscopy techniques. The broadband pump-probe diffuse reflectance spectroscopy technique facilitates monitoring the ultrafast processes in opaque CBTSSe films deposited on Mo-coated glass substrates, similar to the configuration found in functional devices. We spectrally resolve a sharp ground-state bleaching (GSB) peak for CBTSSe films, formed around the band edge transition, which is spectrally narrower than the GSB and stimulated emission in corresponding CZTSSe films. The presence of sharp electronic transitions is further deduced from the ensemble pump-probe spectroscopy and steady-state UV-vis diffuse reflectance spectra. Furthermore, using pump-probe diffuse reflectance scanning microscopy, we monitor the charge carrier formation and excited state pattern within the film grains at few hundred nanometer resolution and localize the kinetics of photogenerated carriers in each grain. The unique sensitivity of pump-probe microscopy and sharp electronic transitions allow for detection of small S/Se stoichiometry variations, Δ x ≤ 0.3, in CBTSSe grains-i.e., features that are largely unresolved for ensemble spectroscopy or luminescence measurements. By noting the sharp band edge transition, we show that the band tailing issue (prevalent for CZTSSe) is largely resolved for CBTSSe; however, other issues may remain, such as deep defects and fast carriers relaxations, which may still impact the photocurrent and open circuit voltage of the CBTSSe devices/films examined.

Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy.

Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and -690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded.

A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials.

Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor-based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye-sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I(3)(-)/I(-) redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)(2)](3+/2+)(PF(6))(3/2) as redox mediator in combination with a cyclopentadithiophene-bridged donor-acceptor dye (Y123), adsorbed on TiO(2), yielded a power conversion efficiency of over 10% at 100 mW cm(-2). This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I(3)(-)/I(-) redox shuttle.

Dynamics and mechanisms of interfacial photoinduced electron transfer processes of third generation photovoltaics and photocatalysis.

Photoinduced electron transfer (PET) across molecular/bulk interfaces has gained attention only recently and is still poorly understood. These interfaces offer an excellent case study, pertinent to a variety of photovoltaic systems, photo- and electrochemistry, molecular electronics, analytical detection, photography, and quantum confinement devices. They play in particular a key role in the emerging fields of third-generation photovoltaic energy converters and artificial photosynthetic systems aimed at the production of solar fuels, creating a need for a better understanding and theoretical treatment of the dynamics and mechanisms of interfacial PET processes. We aim to achieve a fundamental understanding of these phenomena by designing experiments that can be used to test and alter modern theory and computational modeling. One example illustrating recent investigations into the details of the ultrafast processes that form the basis for photoinduced charge separation at a molecular/bulk interface relevant to dye-sensitized solar cells is briefly presented here: Kinetics of interfacial PET and charge recombination processes were measured by fs and ns transient spectroscopy in a heterogeneous donor-bridge-acceptor (D-B-A) system, where D is a Ru(II)(terpyridyl-PO3)(NCS)3 complex, B an oligo-p-phenylene bridge, and A nanocrystalline TiO2. The forward ET reaction was found to be faster than vibrational relaxation of the vibronic excited state of the donor. Instead, the back ET occurred on the micros time scale and involved fully thermalized species. The D-A distance dependence of the electron transfer rate was studied by varying the number of p-phenylene units contained in the bridge moiety. The remarkably low damping factor beta = 0.16 angstroms(-1) observed for the ultrafast charge injection from the dye excited state into the conduction band of TiO2 is attributed to the coupling of electron tunneling with nonequilibrium vibrations redistributed on the bridge, giving rise to polaronic transport of charges from the donor ligand to the acceptor solid oxide surface.

Photoinduced interfacial electron transfer and lateral charge transport in molecular donor-acceptor photovoltaic systems.

Nanostructured liquid/solid and solid/solid bulk heterojunctions designed for the conversion of solar energy offer ideal models for the investigation of light-induced ET dynamics at surfaces. Despite significant study of processes leading to charge generation in third-generation solar cells, a conclusive picture of the photophysics of these photovoltaic converters is still missing. More specifically searched is the link between the molecular structure of the interface and the kinetics of surface photoredox reactions. Fundamental scientific issues in this field are addressed by the research project undertaken in the frame of the NCCR MUST endeavor, an outline of which is given here.

Enhanced electron collection efficiency in dye-sensitized solar cells based on nanostructured TiO(2) hollow fibers.

Nanostructured TiO(2) hollow fibers have been prepared using natural cellulose fibers as a template. This cheap and easily processed material was used to produce highly porous photoanodes incorporated in dye-sensitized solar cells and exhibited remarkably enhanced electron transport properties compared to mesoscopic films made of spherical nanoparticles. Photoinjected electron lifetime, in particular, was multiplied by 3-4 in the fiber morphology, while the electron transport rate within the fibrous photoanaode was doubled. A nearly quantitative absorbed photon-to-electrical current conversion yield exceeding 95% was achieved upon excitation at 550 nm and a photovoltaic power conversion efficiency of 7.2% reached under simulated AM 1.5 (100 mW cm(-2)) solar illumination.