The explicit role of interfacial hydration during polyethylene glycol induced lipid fusion: a THz spectroscopic investigation

While the phenomenon of excipient mediated membrane fusion has been studied widely, the inherent role of interfacial hydration involved in the process has mostly remained unaddressed. Here we report the experimental validation of the fact that PEG-induced membrane fusion is associated with the dehydration of the membrane(s). We explore the explicit hydration behavior at three different lipids (DOPC, POPC and DPPC) membranes with different aliphatic tails as they undergo fusogenic transition in the presence of PEG of average molecular weight of 4000 using THz-FTIR spectroscopy in the frequency window of 1.5-13.5 THz. Dynamic light scattering and electron microscopic measurements confirm the formation of different intermediate steps of the liposomes during the fusion process: bilayer aggregation, destabilization and finally lipid fusion. We observe that membrane hydration follows a systematic trend with the lipid specificity as the fusion process sets in.

TMDC ternary alloy-based triboelectric nanogenerators with giant photo-induced enhancement

Multifunctional self-powered energy harvesting devices have attracted significant attention for wearable, portable, IoT and healthcare devices. In this study, we report transition metal dichalcogenide (TMDC) ternary alloy (Mo0.5W0.5S2)-based self-powered photosensitive vertical triboelectric nanogenerator (TENG) devices, where the ternary alloy functions both as a triboelectric layer and as a photoabsorbing material. The scalable synthesis of the highly crystalline Mo0.5W0.5S2 ternary alloy can overcome the limitations of binary TMDCs (MoS2, WS2) by utilizing its superior optical characteristics, enabling this semiconductor-based TENG device to simultaneously exhibit photoelectric and triboelectric properties. Benefiting from visible light absorption, this vertical TENG device generates higher triboelectric outputs and exhibits excellent power harvesting properties under visible light illumination. The open circuit voltage and short circuit currents of the devices under illumination (410 nm, 525 μW cm-2) are enhanced by 62% and 253%, respectively, while in the darkness, a very high photoresponsivity of ∼45.5 V mW-1 (voltage mode) is exhibited, indicating the superior energy harvesting potential under ultralow illumination. Furthermore, the energy harvesting ability from regular human activities and the operation as artificial e-skin expands the multi-functionality of this TENG device, paving a pathway for simultaneous mechanical and photonic energy harvesting with self-powered sensing.

Sugar Molecules Inhibit Insulin Aggregation: A Decisive Role Being Played by the Protein Solvation Energetics

Insulin plays vital roles in controlling blood sugar level in the human body. However, it sometimes aggregates during the storage, and its efficacy (on the treatment of diabetes II disease) reduces significantly. So, understanding the insulin aggregation could help in long-term storage. Here we investigate the amyloid growth of human insulin protein in the presence of sugar molecules and observe that glucose and sucrose delay the insulin aggregation, the effect being systematically sugar dependent. We then investigate protein hydration during the aggregation process using terahertz spectroscopy, as the hydration plays a pioneering role in maintaining biological systems. Our study infers that the water network changes systematically with protein conformations and solvation entropy-enthalpy balance plays a decisive role in the aggregation process.

Impact of hydrophobicity on local solvation structures and its connection with the global solubilization thermodynamics of amphiphilic molecules

The relationship between the local solvation structures and global thermodynamics, specifically in the case of amphiphilic molecules, is a complex phenomenon and is not yet fully understood. With the prior knowledge that local solvation structures can impose a significant impact on the overall solvation process, we here combine THz spectroscopic analysis with MD simulations to investigate the impact of the altered hydrophobicity and polarity of amphiphilic solute molecules on the local solvation configurations. We use two water soluble alcohols: ethanol (EtOH) and its fluorinated counterpart, 2,2,2-trifluoroethanol (TFE), as model solutes. Our study is aimed to determine the relative abundance of different hydrogen bonded conformers and to establish a correlation between the spectral signatures (as obtained from THz spectroscopic measurements) and microscopic solute-solvent interactions associated with the local solvation structures (as obtained from MD simulations). Finally, we estimate the possible energetic parameters associated with the alcohol solubilization process. We found that while both the alcohols are completely water soluble, they receive a contrasting solvation energy share in terms of entropy and enthalpy. We understand that these findings are not limited to the specific system studied here but can be broadly extrapolated to other amphiphilic aqueous solutions.

Solvation Plays a Key Role in Antioxidant-Mediated Attenuation of Elevated Creatinine Level: An In Vitro Spectroscopic Investigation

An elevated level of creatinine (CRN) is a mark of kidney ailment, and prolonged retention of such condition could lead to renal failure, associated with severe ischemia. Antioxidants are clinically known to excrete CRN from the body through urine, thereby reducing its level in blood. The molecular mechanism of such an exclusion process is still illusive. As the excretion channel is urine, solvation of the solute is expected to play a pivotal role. Here, we report a detailed time-domain and frequency-domain terahertz (THz) spectroscopic investigation to understand the solvation of CRN in the presence of two model antioxidants, mostly used to treat elevated CRN level: N-Acetyl-l-cysteine (NAC) and ascorbic acid (ASC). FTIR spectroscopy in the mid-infrared region and UV absorption spectroscopy measurements coupled with quantum chemical calculations [at the B3LYP/6-311G++(d,p) level] reveal that both NAC and ASC form HBonded complexes with CRN and rapidly undergo a barrier-less proton transfer process to form creatinium ions. THz measurements provide explicit evidence of the formation of highly solvated complexes compared with bare CRN, which eventually enables its excretion through urine. These observations could provide a foundation for designing more beneficial drugs to resolve kidney diseases..

Electrochemical, surface morphological and computational evaluation on carbohydrazide Schiff bases as corrosion inhibitor for mild steel in acidic medium

Anticorrosion and adsorption behaviour of synthesized carbohydrazide Schiff bases, namely (Z)-N'-(4-hydroxy-3-methoxybenzylidene)-6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carbohydrazide(MBTC) and (Z)-N'-(3,4-dichlorobenzylidene)-6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carbohydrazide (CBTC) was examined for mild steel (MS) in 15% HCl medium. The corrosion inhibition study was performed by using gravimetric, thermodynamic, electrochemical and theoretical studies including density functional theory (DFT), molecular dynamic simulation (MDS) and Monte Carlo simulations (MCS). The outcomes in terms of corrosion inhibition efficiency using electrochemical impedance spectroscopy (EIS) method at 303 K and 150 ppm concentration were 96.75% for MBTC and 95.14% for CBTC. Both inhibitors adsorbed on the MS surface through physical as well as chemical adsorption and followed the Langmuir isotherm. The mixed-type nature of both inhibitors was identified by polarization results. Surface analysis was done using FESEM, EDX, AFM and XPS studies and results showed that a protective layer of inhibitor molecules was developed over the surface of MS. The results of DFT, MCS and MDS are in accordance with experimental results obtained by weight loss and electrochemical methods.

Superior piezoelectric performance of chemically synthesized transition metal dichalcogenide heterostructures for self-powered flexible piezoelectric nanogenerator

In addition to the superior electrical and optoelectronic attributes, ultrathin two-dimensional transition metal dichalcogenides (TMDCs) have evoked appreciable attention for their piezoelectric properties. In this study, we report, the piezoelectric characteristics of large area, chemically exfoliated TMDCs and their heterostructures for the first time, as verified by piezoelectric force microscopy measurements. Piezoelectric output voltage response of the MoS2-WSe2 heterostructure piezoelectric nanogenerator (PENG) is enhanced by ~47.5% if compared with WSe2 and ~ 29% if compared to MoS2 PENG, attributed to large band offset induced by heterojunction formation. This allows the scalable fabrication of self-powered energy harvesting piezoelectric nanogenerators, which can overcome the various shortcomings of complicated synthesis processes, complex fabrication steps, low yield and poor stability. The fabricated flexible, self-powered MoS2-WSe2 heterostructure nanogenerator exhibits piezoelectric output ~46 mV under a strain of ~0.66% yielding a power output ~12.3 nW, which offers better performance than other two-dimensional material based piezoelectric devices and also reveals the ability of bio-mechanical energy harvesting. This cost effective approach to fabricate eco-friendly MoS2-WSe2 based fatigue free, superior performance piezoelectric-nanogenerators can be utilized to evolve flexible energy harvesting devices and may also be attractive as a self-powered, smart wearable sensor devices.

Thermo-Resistive Phase Behavior of Trivalent Ion-Induced Microscopic Protein-Rich Phases: Correlating with Ion-Specific Protein Hydration

Proteins, in the presence of trivalent cations, exhibit intriguing phase behavior which is contrasting compared to mono- and divalent cations. At room temperature (RT), trivalent cations induce microscopic liquid-liquid phase separation (LLPS) in which a protein-rich phase coexists with a dilute phase. The critical solution temperature related phenomena in these complex fluids are well studied; however, such studies have mostly been restricted below the denaturation temperature (T_M) of the protein(s) involved. Here, we probe the phase behavior of bovine serum albumin (BSA) incubated at 70 °C (>T_M) in the presence of Na⁺, Mg²⁺, La³⁺, Y³⁺, and Ho³⁺ ions. BSA in the presence of mono- and bivalent ions forms an intense gel phase at 70 °C; however, the trivalent salts offer remarkable thermal resistivity and retain the fluid LLPS phase. We determine the microscopic phase behavior using differential interference contrast optical microscopy, which shows that the LLPS droplet structures in the M³⁺ ion-containing protein solutions prevail upon heating, whereas Mg²⁺ forms composed cross-linking gelation upon thermal incubation. We probe the interior environment of the protein aggregates by ps-resolved fluorescence anisotropy measurements using 8-anilino-1-naphthalenesulfonic acid (ANS) as an extrinsic fluorophore. It reveals that while the LLPS phase retains the rotational time constants upon heating, in the case of gelation, the immediate environment of ANS gets significantly perturbed. We investigate the explicit protein hydration at RT as well as at T > T_M using the ATR THz-FTIR (1.5-22.5 THz) spectroscopy technique and found that hydration shows strong ion specificity and correlates the phase transition behavior.

Interfacial Structure and Electrostatics Related to Solute Activity in a Model Anionic-Surfactant/Polymer Self-Assembly

Polymer/surfactant composites are used in industry as an excipient for water-insoluble solutes. Such enhanced dissolution ability of composite media is related to the spontaneous formation of pre-micellar polymer surfactant aggregates (PS) at a magnitude of order lower than the surfactant critical micelle concentration in water. Combining electrochemical and spectroscopic studies, we investigate the microscopic interfacial structure (i.e., interface electrostatics and surface polarity) of PS formed in composite media. We establish that in a composite system, a mere change in the polymer concentration at a fixed surfactant concentration makes possible to regulate the counter-ion binding ability, surface potential, surface charge density, packing and surface polarity of the PS interface. Our study shows that the higher dissolution of water-insoluble nonionic solutes in composite media is driven by the depressing of surface charge density and polarity of the PS interface. A similar modulation of the PS interface acts as a barrier for the passive relocation of water-soluble charged solutes into the PS pseudo-phase. The time-resolved fluorescence anisotropy study allows us to underline the effect of surface charge modulation on the dynamical aspects of solutes at the PS interface.

Trivalent cation-induced phase separation in proteins: ion specific contribution in hydration also counts

Multivalent (specifically trivalent) metal ions are known to induce microscopic phase separation (commonly termed as liquid-liquid phase separation (LLPS)) in negatively charged globular proteins even at ambient temperatures, the process being mostly driven by protein charge neutralization followed by aggregation. Recent simulation studies have revealed that such self-aggregation of proteins is entropy driven; however, it is associated with a solvation effect, which could as well be different from the usual notion of hydrophobic hydration. In this contribution we have experimentally probed the explicit change in hydration associated with ion-induced LLPS formation of a globular protein bovine serum albumin (BSA) at ambient temperature using FIR-THz FTIR spectroscopy (50-750 cm^-1; 1.5-22.5 THz). We have used ions of different charges: Na⁺, K⁺, Ca²⁺, Mg²⁺, La³⁺, Y³⁺, Ho³⁺ and Al³⁺. We found that all the trivalent ions induce LLPS; the formation of large aggregates has been evidenced from dynamic light scattering (DLS) measurements, but without perturbing the protein structure as confirmed from circular dichroism (CD) measurements. From the frequency dependent absorption coefficient (α(ν)) measurements in the THz frequency domain we estimate the various stretching/vibrational modes of water and we found that ions, forming LLPS, produce definite perturbation in the overall hydration, the extent of which is ion specific, invoking the definite role of hydrophilic (electrostatic) hydration of ions in the observed LLPS process.

Addition of cholesterol alters the hydration at the surface of model lipids: a spectroscopic investigation

Cholesterol is known to modify the phase behavior of model lipid membranes as it makes phospholipid bilayers more structured. Simulation results have shown that the addition of cholesterol allows more bulk-like water to protrude into phospholipid interfaces. However, such claims have not yet been verified experimentally. We have investigated the alteration in the hydrogen bond network structure of water at the surface of two model phospholipids DOPC and DOPG as cholesterol is added into these using ATR-FTIR spectroscopy in the FIR-THz region. Our measurements and analysis led us to probe the collective H-bond network explicitly at the lipid surface. A detailed principal component analysis of the measured data concludes that the water-water H-bond vibration dynamics gets slower at the lipid surface as compared to bulk water, the effect being more prominent in the case of the charged phospholipid, DOPG. However, as cholesterol is added and more bulk-like water protrudes into the liposome interface, the H-bond vibration gets weaker and correspondingly the dynamics gets accelerated.

The inner hydration in surfactant/cholesterol vesicles differs from the outer one: a spectroscopic investigation

Vesicles contain two aqueous regions: inner core and outer-to-bulk . It has remained an open question whether hydration behaviour in the inner core differs from the outer-to-bulk region, mostly owning to the inability of the conventional spectroscopic techniques to deconvolute the contribution from these two regions. We, using THz-FTIR spectroscopy (1.5-13.5 THz) experimentally probe the inner hydration of three differently charged surfactant/cholesterol vesicles composed of SDS, CTAB and Brij 30. Both dynamic light scattering (DLS) and atomic force microscopy (AFM) measurements affirm the transition from micelles to vesicles as cholesterol is added into surfactant solutions. FTIR measurements show that hydration behaviour changes significantly as micelles are converted into vesicles, the change been exclusively caused due to the formation of an inner core . Our measurements on the hydrogen bond stretch and librational motion of the inner hydration show distinct features compared to the overall hydration, which in turn is found to be surfactant type and cholesterol concentration dependent.

Nonpolar hydrophobic amino acids tune the enzymatic activity of lysozyme

We have used five different hydrophobic L-amino acids (Gly, Ala, Val, Leu, Ile) as molecular crowders to investigate their role on the enzymatic activity of lysozyme towards Micrococcus lysodeikticus (M. lys.)cell as substrate. We found that except Ile, all other amino acids show a bell like profile of catalytic efficiency (k_cat/K_m) with their increasing concentration whereas for Ile, the value is gradually increasing. The trend of activation energy (E_a) is also well correlated with the catalytic efficiency of lysozyme. At low concentration of amino acids, soft interaction predominates whereas at higher concentration range, excluded volume, viscosity, hydrophobicity combinedly decrease the activity of lysozyme.

Molecular Insight into Dye-Surfactant Interaction at Premicellar Concentrations: A Combined Two-Photon Absorption and Molecular Dynamics Simulation Study

Both electrostatic and hydrophobic interactions play pivotal roles in ligand-surfactant binding interaction, especially for ionic surfactants. While much studies have been reported in the micellar region, less attention has been paid on such interactions at a low (premicellar) surfactant concentration. We here study the interaction between the cationic dye rhodamine 6G (R6G) with surfactants of different charge types: anionic SDS, cationic CTAB, and nonionic Tx 100 using absorption and emission spectroscopy. We identify that R6G forms dimeric aggregates at a premicellar concentration of SDS. Formation of aggregates is also confirmed from classical simulation measurements. CTAB and Tx 100 do not form any such aggregate, presumably owing to unfavorable electrostatic interactions. For a molecular-level understanding, we perform two-photon absorption (TPA) spectroscopy for the same systems. TPA allows us to calculate the two-photon absorption cross section and subsequently the change in the dipole moment (Δμ) between ground and excited states of the dye. We calculate the Δμ and observe that it passes through a maximum at a surfactant concentration half of the critical micelle concentration of SDS. This observation imparts support to earlier quantum mechanical calculation, which infers deviation from the parallel orientation of the dye during surfactant-induced aggregation. We extended our measurements and varied the carbon chain length of the anionic surfactant, and we found that all of them exhibit a maximum in Δμ, while their relative magnitude is dependent on the surfactant carbon chain length.

High-Responsivity Gate-Tunable Ultraviolet-Visible Broadband Phototransistor Based on Graphene-WS2 Mixed-Dimensional (2D-0D) Heterostructure

Recent progress in the synthesis of highly stable, eco-friendly, cost-effective transition-metal dichalcogenide (TMDC) quantum dots (QDs) with their broadband absorption spectra and wavelength selectivity features have led to their increasing use in broadband photodetectors. With the solution-based processing, we demonstrate a superlarge (∼0.75 mm²), ultraviolet-visible (UV-vis) broadband (365-633 nm) phototransistor made of WS₂ QDs-decorated chemical vapor deposited (CVD) graphene as the active channel with extraordinary stability and durability under ambient conditions (without any degradation of photocurrent until 4 months after fabrication). Here, colloidal zero-dimensional (0D) WS₂ QDs are used as the photoabsorbing material, and graphene acts as the conducting channel. A high photoresponsivity (3.1 × 10² A/W), moderately high detectivity (∼8.9 × 10⁸ Jones), and low noise equivalent power (∼9.7 × 10^-11 W/Hz^0.5) are obtained at a low bias voltage (V_ds = 1 V) at an illumination of 365 nm with optical power as low as ∼0.8 μW/cm², which can be further tuned by modulating the gate bias. While comparing the photocurrent between two different morphologies of WS₂ [QDs and two-dimensional (2D) nanosheets], a significant enhancement of photocurrent is observed in the case of QD-based devices. Ab initio density functional theory (DFT)-based calculations further support our observation, revealing the role of quantum confinement in enhanced photoresponse. Our work reveals a strategy toward developing a scalable, cost-effective, high-performance hybrid mixed-dimensional (2D-0D) photodetector with graphene-WS₂ QDs for next-generation optoelectronic applications.

Excipients Do Regulate Phase Separation in Lysozyme and Thus Also Its Hydration

While the liquid-liquid phase separation (LLPS) process in proteins has been studied in great detail, it has not been widely explored how the associated protein hydration changes during the process and how crucial its role is in the process itself. In this contribution, we experimentally explore the alteration of lysozyme hydration during its LLPS process using attenuated total reflection (ATR)-FTIR spectroscopy in the THz frequency region (1.5-21 THz). Additionally, we explore the role of excipients (l-arginine, sucrose, bovine albumin (BSA), and ubiquitin (Ubi)) in regulating the process and found that, while sucrose stabilizes the LLPS, BSA inhibits it. The effect of Arg in the LLPS is subtle, and that of Ubi is concentration dependent. We made a detailed analysis of the hydration profile of Lys in the presence of these excipients and observe that a change in hydration in terms of H-bond making/breaking is a definite signature regulating the process.

2D WS2 embedded PVDF nanocomposites for photosensitive piezoelectric nanogenerators with a colossal energy conversion efficiency of ∼25.6

Benefiting from the advantages of low cost, light weight and mechanical flexibility, piezoelectric nanogenerators have the potential for application in renewable energy harvesting from various unexplored sources. Here, we report the demonstration of the record efficiency of flexible piezoelectric nanogenerators (PENG) using composites of polyvinylidene fluoride (PVDF) and chemically exfoliated tungsten disulfide (WS₂) nanosheets, which are found to be strongly photosensitive, making them attractive for self-powered optical devices. The presence of two-dimensional (2D) WS₂ nanosheets in the PVDF matrix plays a dual role in enhancing the nucleation of the electroactive β-phase as well as inducing strong photosensitivity in the nanocomposite. The PVDF-WS₂ composed flexible device is able to produce an enormously high output voltage of ∼116 V (for an impact of 105 kPa) and a piezoelectric energy conversion efficiency of ∼25.6%, which is the highest among the reported values for PVDF-2D material based self-poled piezoelectric nanogenerators. This self-poled piezo-phototronic device exhibits strain-dependent photocurrent at zero bias and exhibits a responsivity of 6.98 × 10^-3 A W^-1 at 0.75% strain under the illumination of 410 nm. The fabricated PENG is also able to harvest energy from routine human activities (finger tapping, writing on paper, mouse clicking, etc.) and movement of human body parts. These results open up a new horizon in piezo-phototronic materials through the realization of photosensitive multifunctional PENGs, which can be scaled up for fabricating compact, high performance, portable and self-powered wearable electronic devices for smart sensor applications.

Correlating solvation with conformational pathways of proteins in alcohol-water mixtures: a THz spectroscopic insight

Water, being an active participant in most of the biophysical processes, is important to trace how protein solvation changes as its conformation evolves in the presence of solutes or co-solvents. In this study, we investigate how the secondary structures of two diverse proteins - lysozyme and β-lactoglobulin - change in the aqueous mixtures of two alcohols - ethanol and 2,2,2-trifluoroethanol (TFE) using circular dichroism measurements. We observe that these alcohols change the secondary structures of these proteins and the changes are protein-specific. Subsequently, we measure the collective solvation dynamics of these two proteins both in the absence and in the presence of alcohols by measuring the frequency-dependent absorption coefficient (α(ν)) in the THz (0.1-1.2 THz) frequency domain. The alcohol-water mixtures exhibit a non-ideal behaviour with the highest absorption difference (Δα) obtained at X_alcohol = 0.2. The protein solvation in the presence of the alcohols shows an oscillating behaviour in which Δα_protein changes with X_alcohol. Such an oscillatory behaviour of protein solvation results from a delicate interplay between the protein-water, protein-alcohol and water-alcohol associations. We attempt to correlate the various structural conformations of the proteins with the associated solvation.

Effect of Surfactant Tail Length on the Hydroxypropyl Cellulose-Mediated Premicellar Aggregation of Sodium n-Alkyl Sulfate Surfactants

Polymer/surfactant composites have emerged as a subject of interest for their diverse applications. The improved solution properties in polymer/surfactant composites have been correlated to the formation of premicellar surfactant aggregate-polymer complexes (PS) at a surfactant concentration well below their critical micelle concentrations. Using different physicochemical and spectroscopic techniques here we have studied PS formed by hydroxypropyl cellulose, a nonionic-biocompatible polymer, and alkyl sulfate surfactants of different tail lengths. Our study shows that an increase in surfactant tail length eases PS formation and enhances PS-induced polymer cross-linking and, correspondingly, solution viscosity. PS consisting of shorter tail surfactants and those with longer tail surfactants differ microscopically as the former offers more polar interior than the later as evidenced from fluorescence measurements. Our study establishes that shorter tail surfactants intend to stay loosely packed inside PS and allow larger water penetration, which creates a relatively polar hydrophobic core compared to the PS with longer tail surfactants. The stronger packing of PS with longer tail surfactants is an outcome of favorable interaction between polymer polar groups and surfactant headgroups, which further creates strongly hydrogen-bonded water in their hydration shell.

Alteration of water absorption in the THz region traces the onset of fibrillation in proteins

Using terahertz spectroscopy, we established the alteration of the collective hydration of water during the fibrillation process (native → intermediate → fibril) of a model protein bovine serum albumin. This label-free study concludes that water dynamics change systematically with protein conformational changes as it experiences a hydrophobic environment during the initial protein unfolding process, followed by the release of bound water during oligomerization and finally the hydrophobic interior of the fibril.

Two-dimensional graphitic carbon nitride nanosheets: a novel platform for flexible, robust and optically active triboelectric nanogenerators

We report on the characteristics of mechanically flexible, stable and photoactive triboelectric nanogenerators based on two-dimensional graphitic carbon nitride (g-C₃N₄) nanosheets. The performance of nanogenerator devices has been studied with varying frictional surfaces (such as polypropylene, aluminium oxide, Teflon and polyethylene terephthalate). Energy band diagrams have been used to explain the mechanism of triboelectric charge transfer in pristine and doped g-C₃N₄, with the former showing better characteristics. An optimized device has been found to be responsive to external stimuli to generate an output voltage of 10 V upon simple biomechanical impulses. To demonstrate the efficacy for practical applications of g-C₃N₄-based triboelectric nanogenerators, output voltages have been recorded for different common activities like walking, water showering, using as a writing/drawing pad, etc. Repetitive finger tapping on a device could charge a capacitor to as high as 55 V within ∼50 s, while that under UV illumination is found to be much faster (∼14 s) due to photoinduced carrier generations in g-C₃N₄. The exhibition of a superior photoresponsivity of ∼117 V W^-1 under UV illumination demonstrates the dual functionality of g-C₃N₄-based triboelectric devices as a nanogenerator as well as an active flexible photosensor, which is hitherto unreported. Excellent mechanical flexibility, stability and photoinduced enhancement of output characteristics make g-C₃N₄ an attractive candidate for nanogenerator devices for future applications.

Self-powered flexible photodetectors based on Ag nanoparticle-loaded g-C3N4 nanosheets and PVDF hybrids: role of plasmonic and piezoelectric effects

Here we demonstrate novel self-powered photodetection using silver (Ag) nanoparticle-loaded two-dimensional graphitic carbon nitride (g-C₃N₄) nanosheets triggered by poly-vinylidene fluoride (PVDF)-based flexible piezoelectric nanogenerators. A self-poled PVDF-based nanogenerator has been obtained upon exploiting pristine g-C₃N₄ nanosheets as a filler material within the PVDF matrix. The fabricated nanogenerator devices are found to be highly efficient in generating the maximum voltage of ∼2.3 V and maximum power ∼110 μWatt/cm², upon finger tapping. Further, the integration of an additional layer of plasmonic Ag nanoparticle-loaded g-C₃N₄ nanosheets, has led to a significant enhancement of photoresponse. The hybrid plasmonic nanogenerator (with a strain of ∼0.021%) has resulted in self-powered photodetection with a photo-to-dark current ratio of ∼60, as compared to the unstrained device (∼2.0). In contrast to the usual behaviour (positive photoresponse), the exposure of an ultraviolet light lowers the output current indicating a negative photoresponse reported for the first time in such a system. The origin of such negative photoresponse has been attributed to the screening of piezopotential of PVDF by photogenerated carriers of g-C₃N₄ nanosheets. On the other hand, visible light-induced positive photoresponse has originated from the increment in the current, indicating the useful role of Ag nanoparticles in plasmon-induced hot electron transfer process.

Connection of large amplitude angular jump motions with temporal heterogeneity in aqueous solutions

It has now been established that large angular jumps do take place when a rotating water molecule exchanges its hydrogen bond (H-bond) identity. This motion differs from the small angular diffusional steps occurring within short time intervals which define the 'Debye diffusion model' of water dynamics. We intend to investigate whether these two processes do eventually complement each other. In this present investigation the orientational dynamics of water in its mixture with a small hydrophobic molecule 1,2-dimethoxy ethane (DME) is studied microscopically using the all-atom classical molecular dynamics (MD) simulation technique. We found that the reorientational motions of water molecules are governed by continuous making and breaking of intermolecular H-bonds with their partners. We characterise these H-bond reorientation motions with the description of the "large amplitude angular jump model" and explore the coupling between the rotational and translational motions. By following the trajectories of each molecule in the solutions we describe the orientational dynamics of liquid water with a 'continuous time random walk' (CTRW) approach. Finally, we explore the diffusivity distribution through the jump properties of the water molecules, which successfully leads to the inherent transient heterogeneity of the solutions. We observe that the heterogeneity increases with increasing DME content in the mixtures. Our study correlates the coupling between rotational and translational motions of water molecules in the mixtures.

Size-dependent optical properties of MoS2 nanoparticles and their photo-catalytic applications

While two-dimensional (2D) layered MoS₂ nanosheets have been extensively studied owing to their fascinating optoelectronic properties, less attention has been paid to the corresponding zero-dimensional nano-crystals. In this contribution, we report the efficacy of MoS₂ nanocrystals for their size tunable properties for optical and photocatalytic applications. We have synthesized differently sized (10-70 nm) crystalline, hexagonal 2H-MoS₂ nanoparticles (NPs) dispersed in DMF solvent using a simple exfoliation technique. Synthesized NPs are found to exhibit size-dependent optical properties and excitation-dependent fluorescence characteristics in the visible region, which are otherwise not observed in bulk or 2D MoS₂ layers. Size tunable bandgap and broad absorbance and emission spectrum covering the visible range could be exploited in the fabrication of various opto-electronic devices. Charge carrier emission dynamics of differently sized MoS₂ NPs are investigated using time correlated single photon counting (TCSPC) spectroscopic technique. We found two time components, one in the order of several hundreds of ps, which arises due to the radiative recombination of charge carriers, while the other one is of the order of a few ns, which emanates from the defect states of MoS₂ NPs. The average time constants are found to decrease with increase in particle size. A noticeable photocatalytic activity of the synthesized MoS₂ NPs under visible light illumination for the degradation of brilliant green dye is also demonstrated for the first time and the effect of size variation of NPs in the dye degradation process is reported.

Polyethylene glycols affect electron transfer rate in phenosafranin-DNA complex

Long distance electron transfer (ET) between small ligands and DNA is a much studied phenomenon and is principally believed to occur through electron (or hole) hopping. Several studies have been carried out in aqueous environments while in real biological milieu the DNA molecules experience a more dense and heterogeneous environment containing otherwise indifferent molecular crowders. It is therefore expected that the ET could get modified in the presence of crowding agent and to investigate that we have made elaborate studies on steady state and time-resolved (picosecond (ps) and femtosecond (fs)-resolved) emission properties of a phenosafranine (PSF) intercalated to calf thymus (CT) DNA in the presence of ethylene glycol (EG) and polyethylene glycols (PEG) of different chain lengths (PEG 200, 400 and 1000). The emission of PSF gets considerably quenched when intercalated to DNA; the quenching is released when PEGs are added into it. The structural integrity of the CT DNA has been established using circular dichroism spectroscopy. CD measurements have evidenced only marginal changes in the DNA structure upon the addition of PEGs. ps-Resolved fluorescence measurements show significant decrease in the contribution of the DNA induced quenched time-constant of PSF upon the addition of PEGs, however, fs-resolved measurements show less noticeable changes in the time constants. Our study shows that the electron hopping rate through the guanine base in DNA core remains unaffected whereas the 'through space' electron transfer process does get affected in the presence of molecular crowders.

Thermal stability modulation of the native and chemically-unfolded state of bovine serum albumin by amino acids

Cells are crowded with various cosolutes including salts, osmolytes, nucleic acids, peptides and proteins.

Complexation and fluorescence behavior of proflavin with chemically engineered amine capped carbon nanodots and its subsequent release into DNA environments

Amine capped carbon dots are prepared by pyrolysis of citric acid. Probable excited-state interactions between PF and CNDs have been studied. A controlled release of PF into ctDNA by CNDs shows their utility as an efficient drug delivery agent.

Soft interaction and excluded volume effect compete as polyethylene glycols modulate enzyme activity

Polyethylene glycols (PEGs) can either preferentially bind to biomolecules or exert excluded volume effect depending upon their chain length and concentration. We have studied the effect of ethylene glycol (EG) and PEGs of different chain lengths (M_n 400 and 4000) on the enzyme efficiency of hen-egg-white lysozyme (HEWL) on Micrococcus lysodeikticus (M. Lys.) cell. The activity shows a bell-like profile as the turnover number increases from ~1.3 × 10⁵ s^-1 M^-1 in water to ~1.7 × 10⁵ s^-1 M^-1 in presence of 2% PEG-400 beyond which it decreases to ~0.7 × 10⁵ s^-1 M^-1 at 20% PEG-400. Solvent polarity, excluded volume effect, soft nonspecific interactions and structural flexibility are found to be the competing factors which govern the overall enzyme activity as evidenced from circular dichroism (CD) and fluorescence measurements. Thermal unfolding temperature (T_m) of HEWL also shows a bell-shaped profile with PEG concentration which establishes possible correlation with its activity. We also observe a minimum in the activation energy barrier for the catalysis at low osmolyte concentrations. The maximum in the enzyme efficiency has been explained on the basis of an optimization between excluded volume effect and soft interaction among the protein and the cosolutes.

Modulation of the Excited-State Proton Transfer Rate of d-luciferin in Mixed Reverse Micellar Systems

The excited-state proton transfer (ESPT) rate of photo-acids in a confined medium depends on several physical parameters of the immediate environment. We introduce a new parameter in the form of charge type at the interface of reverse micellar (RM) systems to modulate the ESPT rate. We investigate the ESPT reaction of d-luciferin in mixed RM systems composed of nonionic polyoxyethylene(5)nonylphenylether (Igepal CO-520) with cationic didodecyldimethylammonium bromide (DDAB) and anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in cyclohexane (Cy) at different mole fractions of Ig (X _Ig) and fixed hydration. ESPT is feeble in AOT RM, whereas it is favorable in the other two RMs. Addition of Ig is observed to facilitate ESPT in AOT RM linearly, whereas in DDAB, it shows a synergistic effect. The various physical parameters of water in the mixed RM water pool have been investigated using dynamic light scattering, Fourier transform infrared, and time-resolved fluorescence spectroscopy measurements to underline the ESPT mechanism in these mixed RMs.

Ionic liquid mediated micelle to vesicle transition of a cationic gemini surfactant: a spectroscopic investigation

In this contribution, we have examined a composition dependent self aggregated structural modification of a catanionic mixture of the surface active ionic liquid (IL) 1-butyl-3-methylimidazolium octyl sulphate and a cationic gemini surfactant (14-5-14) in aqueous medium. We have observed that the hydrodynamic diameter of the aggregates increases with increasing IL concentration and microscopic evidence (HRTEM, FESEM, and LCSM) shows the formation of vesicle like aggregates (Dh ≈ 200 nm) at XIL = 0.5. The steady state fluorescence anisotropy of the membrane binding probe DPH shows a micelle to vesicle transition at this composition. The viscosity of the solution shows a peak at XIL = 0.3, indicating the formation of a worm like micelle as an intermediate of the micelle to vesicle transition. The rotational dynamics shows a stiffer surfactant packing in the vesicles compared to the micelles, whereas, the solvation dynamics measurements indicate a higher abundance of bound type water in the vascular medium compared to that for the micelle. The formed vesicles also show stability towards temperature and biomolecules, which can be used for respective applications.

Field-controlled ultrafast magnetization dynamics in two-dimensional nanoscale ferromagnetic antidot arrays

Ferromagnetic antidot arrays have emerged as a system of tremendous interest due to their interesting spin configuration and dynamics as well as their potential applications in magnetic storage, memory, logic, communications and sensing devices. Here, we report experimental and numerical investigation of ultrafast magnetization dynamics in a new type of antidot lattice in the form of triangular-shaped Ni₈₀Fe₂₀ antidots arranged in a hexagonal array. Time-resolved magneto-optical Kerr effect and micromagnetic simulations have been exploited to study the magnetization precession and spin-wave modes of the antidot lattice with varying lattice constant and in-plane orientation of the bias-magnetic field. A remarkable variation in the spin-wave modes with the orientation of in-plane bias magnetic field is found to be associated with the conversion of extended spin-wave modes to quantized ones and vice versa. The lattice constant also influences this variation in spin-wave spectra and spin-wave mode profiles. These observations are important for potential applications of the antidot lattices with triangular holes in future magnonic and spintronic devices.

Efficient terahertz anti-reflection properties of metallic anti-dot structures

We report the use of micrometer-sized copper (Cu) anti-dot structures as a novel terahertz (THz) anti-reflection coating (ARC) material and their superior performance over conventionally used metallic (Cu) thin films. Cu anti-dot structures of two different thicknesses (7 and 10 nm) with varying anti-dot diameters (100, 200, and 300 μm, inter-anti-dot separation fixed at 100 μm) are deposited on silicon substrates by RF magnetron sputtering and e-beam evaporation. The anti-reflection performance of these samples is studied in the frequency range of 0.3-2.2 THz. While continuous metallic (Cu) thin film minimizes the Fabry-Perot (FP) peak, it also suppresses the primary transmission peak, reducing the advantage due to the former effect. On the contrary, the anti-dot arrays reduce both the absolute amplitude of the FP peak and the amplitude ratio (AR) of the FP peak to the primary peak, making them a superior material for ARC applications. The AR can be further manipulated by varying the anti-dot size. A universal conductivity phase-matching condition, which is a prerequisite for the disappearance of the FP peak, is observed in these samples. The enhanced anti-reflection performance promotes these anti-dot structures as an efficient terahertz ARC material.

Enhanced Catalytic Activity of α-Chymotrypsin in Cationic Surfactant Solutions: The Component Specificity Revisited

Enhanced catalytic activity (super activity) of enzymes in the presence of surfactants is of key importance in "micellar enzymology"; such super activity is not very trivial, it is highly system specific, and the mechanism behind the activity enhancement is not always well apprehended. We report the catalytic activity of α-chymotrypsin (CHT) on ala-ala-phe-7-amido-4-methylcoumarin (AMC) in the presence of cationic surfactants of different hydrophobic chain lengths: dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB) and octadecyltrimethylammonium bromide (OTAB). It is observed that in comparison to buffer the catalytic activity of CHT is enhanced 5-fold in premicellar DTAB solutions, while negligible changes are observed in CTAB and OTAB. Activity decreases considerably in the post micellar concentration, specifically for the latter two surfactants. A similar trend is also obtained in another substrate 2-napthyal acetate hydrolysis. Such surfactant specific superactivity is intriguing. The protein's secondary and tertiary structures in the presence of these surfactants are determined using circular dichroism (CD) spectroscopy and it is found that both CTAB and OTAB perturb the protein structure significantly, especially in the post micellar concentrations. DTAB, on the other hand, does not produce noticeable changes in the protein structure. The various pairwise interactions present in the system have been underlined using both steady-state and time-resolved fluorescence spectroscopy. Assuming a three-step kinetics model, we determine the free energy changes of the reaction, and the observations have been discussed in the light of the various interactions among the components.

Terahertz conductivity engineering in surface decorated carbon nanotube films by gold nanoparticles

We report the controllable conductivity of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes with their surface walls decorated by gold nanoparticles (Au NPs) with varying concentration in terahertz (THz) frequency range. Colloidal Au NPs of nominal diameter ∼15  nm are synthesized by the reduction of gold chloride solution using tri-sodium citrate. A simple chemical route is followed to attach Au NPs on the surfaces of both types of carbon nanotubes (CNTs). The attachment of Au NPs on the sidewalls of CNTs is confirmed by UV-visible spectroscopy and scanning electron microscope images. THz spectroscopic measurements are carried out at room temperature in transmission geometry in the frequency range of 0.3-2.0 THz. It is found that the THz conductivity of the surface decorated SWNT composites can either be increased or decreased by ±15% than that of the as-prepared SWNT composites by carefully choosing the Au NP concentration. The conductivity variation is qualitatively explained in terms of carrier trapping potential for low Au NP density, and alternative carrier conduction pathways at higher Au NP density and analyzed with the help of a modified universal dielectric relaxation model.

Effect of Short Chain Poly(ethylene glycol)s on the Hydration Structure and Dynamics around Human Serum Albumin

We report the changes in the hydration dynamics around a globular protein, human serum albumin (HSA), in the presence of two short chain crowding agents, namely poly(ethylene glycol)s (PEG 200 and 400). The change in the network water structure is investigated using FTIR spectroscopy in the far-infrared (FIR) frequency range. Site specific changes are obtained by time-resolved fluorescence spectroscopic technique using the intrinsic fluorophore tryptophan (Trp214) of HSA. The collective hydration dynamics of HSA in the presence of PEG molecules are obtained using terahertz (THz) time domain spectroscopy (TTDS) and high intensity p-Ge THz measurements. Our study affirms a considerable perturbation of HSA hydration beyond a critical concentration of PEG.

Collective hydration dynamics of guanidinium chloride solutions and its possible role in protein denaturation: a terahertz spectroscopic study

The remarkable ability of guanidinium chloride (GdmCl) to denature proteins is a well studied yet controversial phenomenon; the exact molecular mechanism is still debatable, especially the role of hydration dynamics, which has been paid less attention. In the present contribution, we have addressed the issue of whether the collective hydrogen bond dynamics of water gets perturbed in the presence of GdmCl and its possible impact on the denaturation of a globular protein human serum albumin (HSA), using terahertz (THz) time domain spectroscopy (TTDS) in the frequency range of 0.3-2.0 THz. The collective hydrogen bond dynamics is determined by fitting the obtained complex dielectric response in a multiple Debye relaxation model. To compare the results, the studies were extended to two more salts: tetramethylguanidinium chloride (TMGdmCl) and sodium chloride (NaCl). It was concluded that the change in hydration dynamics plays a definite role in the protein denaturation process.

Does the optimum hydrophilic lipophilic balance condition affect the physical properties of mixed reverse micelles? A spectroscopic investigation

Synergism in several physical properties as realized in many mixed surfactant reverse micellar (RM) systems often manifests optimum hydrophilic-lipophilic balance (HLB), interdroplet interaction, or both. Such synergism is often desired for specific applications of RM systems; however, a proper rationale on the effect of such phenomenon imparted on the structure, dynamics, and activity of water molecules in RM waterpool is strongly demanded. In the present contribution we have investigated how the optimum HLB condition of mixed RM composed two nonionic surfactants (Igepal 210 and Igepal 630) affects the physical properties of entrapped water molecules in the RM waterpool. The studied mixed RM exhibits synergistic water solubilization behavior as a function of the mixing ratio with a maximum in solubilization capacity being reached at X(Ig630) = 0.3. Dynamic light scattering (DLS) studies show a bimodal distribution of droplet size in this region, whereas it is monomodal in terminal compositions. Fourier transform infrared spectroscopy (FTIR) study in the 3000-3800 cm(-1) region identifies a linear trend in which the content of "bound" water increases at the expense of the "network" water as the content of the hydrophilic surfactant Igepal 630 is increased in the mixture. Subnanosecond relaxation dynamics of the entrapped water as revealed by the fluoroprobe coumarin 500 corroborates a similar linear trend as observed in the FTIR measurements as the rotational diffusion gets retarded with the increase of ethylene oxide chain length of Igepal. Reaction kinetics of solvolysis of benzoyl chloride reaction, however, does not offer any linear trend as it gets slower in the optimum HLB region, the nonlinearity being a consequence of the distribution of the substrate in the different phases.

EMI shielding and conductivity of carbon nanotube-polymer composites at terahertz frequency

We investigate the shielding effectiveness and complex conductivity of single-walled carbon nanotubes (SWNT) distributed in a polyvinyl alcohol (PVA) matrix in the THz frequency range. SWNTs are dispersed in PVA matrices with varying SWNT content (keeping the thickness of SWNT/PVA film constant) using a slow-drying method, and terahertz time-domain spectroscopy (THz-TDS) is performed at room temperature in transmission geometry in the frequency range of 0.3-2.1 THz. The transmittance spectra show a possible application of SWNT/PVA composites as low-bandpass filters in the THz frequency region. Shielding effectiveness of all the samples is measured, and, at a particular probing frequency, they tend to follow a linear relationship with SWNT weight fraction in the polymer matrices. THz conductivity of the composite system is described in the light of a.c. hopping conduction.