Lateral Flow Assay Biotesting by Utilizing Plasmonic Nanoparticles Made of Inexpensive Metals─Replacing Colloidal Gold.

Nanoparticles (NPs) can be conjugated with diverse biomolecules and employed in biosensing to detect target analytes in biological samples. This proven concept was primarily used during the COVID-19 pandemic with gold-NP-based lateral flow assays (LFAs). Considering the gold price and its worldwide depletion, here we show that novel plasmonic NPs based on inexpensive metals, titanium nitride (TiN) and copper covered with a gold shell (Cu@Au), perform comparable to or even better than gold nanoparticles. After conjugation, these novel nanoparticles provided high figures of merit for LFA testing, such as high signals and specificity and robust naked-eye signal recognition. Since the main cost of Au NPs in commercial testing kits is the colloidal synthesis, our development with the Cu@Au and the laser-ablation-fabricated TiN NPs is exciting, offering potentially inexpensive plasmonic nanomaterials for various bioapplications. Moreover, our machine learning study showed that biodetection with TiN is more accurate than that with Au.

Lateral Flow Assays Biotesting by Utilizing Plasmonic Nanoparticles Made of Inexpensive Metals - Replacing Colloidal Gold.

Nanoparticles (NPs) can be conjugated with diverse biomolecules and employed in biosensing to detect target analytes in biological samples. This proven concept was primarily used during the COVID-19 pandemic with gold NPs-based lateral flow assays (LFAs). Considering the gold price and its worldwide depletion, here we show that novel plasmonic nanoparticles (NPs) based on inexpensive metals, titanium nitride (TiN) and copper covered with a gold shell (Cu@Au), perform comparable or even better than gold nanoparticles. After conjugation, these novel nanoparticles provided high figures of merit for LFA testing, such as high signals and specificity and robust naked-eye signal recognition. To the best of our knowledge, our study represents the 1st application of laser-ablation-fabricated nanoparticles (TiN) in the LFA and dot-blot biotesting. Since the main cost of the Au NPs in commercial testing kits is in the colloidal synthesis, our development with TiN is very exciting, offering potentially very inexpensive plasmonic nanomaterials for various bio-testing applications. Moreover, our machine learning study showed that the bio-detection with TiN is more accurate than that with Au.

Regular Arrays of Rod-Shaped Molecular Photoswitches: Synthesis, Preparation, Characterization, and Selective Photoswitching within Mono- and Bilayer Systems.

We assembled photoresponsive mono- and bilayer systems with well-defined properties from rod-shaped molecules equipped with different photoswitches. Using properly chosen chromophores (diarylethene-based switch and unidirectional light-driven molecular motor), we then selectively targeted layers made of the same types of photoswitches using appropriate monochromatic light. UV-vis analysis confirmed smooth and unrestricted photoisomerization. To achieve this, we synthesized a new class of triptycene-based molecular pedestals adept at forming sturdy Langmuir-Blodgett films on a water-air interface. The films were smoothly transferred to gold and quartz surfaces. Repeated deposition afforded bilayer systems: one layer containing diarylethene-based photoswitches and the other a unidirectional light-driven molecular motor. Structural analysis of both mono- and bilayer systems revealed the molecules to be tilted with carboxylic functions pointing to the surface. At least two different polymorphs differing in monolayer thickness and tilt angle (~40° and ~60°) were identified on the gold surface.

Paramagnetism in Microwave-Synthesized Metal-Free Nitrogen-Doped Graphene Quantum Dots.

Nitrogen-doped graphene quantum dots (NGQDs) have gained significant attention due to their various physical and chemical properties; however, there is a gap in the study of NGQDs' magnetic properties. This work adds to the efforts of bridging the gap by demonstrating the room temperature paramagnetism in GQDs doped with Nitrogen up to 3.26 at.%. The focus of this experimental work was to confirm the paramagnetic behavior of metal free NGQDs resulting from the pyridinic N configuration in the GQDs host. Metal-free nitrogen-doped NGQDs were synthesized using glucose and liquid ammonia as precursors by microwave-assisted synthesis. This was followed by dialysis filtration. The morphology, optical, and magnetic properties of the synthesized NGQDs were characterized carefully through atomic force microscopy (AFM), transmission electron microscopy (TEM)), UV-VIS spectroscopy, fluorescence, X-ray photon spectroscopy (XPS), and vibrating sample magnetometer (VSM). The high-resolution TEM analysis of NGQDs showed that the NGQDs have a hexagonal crystalline structure with a lattice fringe of ~0.24 nm of (1120) graphene plane. The N1s peak using XPS was assigned to pyridinic, pyrrolic, graphitic, and oxygenated NGQDs. The magnetic study showed the room-temperature paramagnetic behavior of NGQDs with pyridinic N configuration, which was found to have a magnetization of 20.8 emu/g.

Visible Light-Assisted Coordination of a Rh(III)-BODIPY Complex to Guanine.

Most photodynamic therapeutics (PDTs) used in cancer treatment require oxygen to work efficiently to terminate cancer cells. These PDTs do not efficiently treat tumors in hypoxic conditions. Rh(III) polypyridyl complexes have been reported to have a photodynamic therapeutic effect in hypoxic conditions when exposed to UV light. UV light can damage tissue and cannot penetrate deep to reach cancer cells. This work proposes the coordination of a BODIPY fluorophore to a rhodium metal center to form a Rh(III)-BODIPY complex that enhances the reactivity of the rhodium under visible light. This complex formation is facilitated with the BODIPY as the highest occupied molecular orbital (HOMO), while the lowest unoccupied molecular orbital (LUMO) is localized on the Rh(III) metal center. Irradiation of the BODIPY transition at ∼524 nm can cause an indirect electron transfer from the orbital of the BODIPY-centered HOMO to the Rh(III)-centered LUMO, populating the dσ* orbital. In addition, photo binding of the Rh complex covalently coordinated to the N (7) position of guanine in an aqueous solution was also observed by mass spectrometry after chloride dissociation upon irradiation with green visible light (532 nm LED). Calculated thermochemistry values of the Rh complex reaction in methanol, acetonitrile, water, and guanine were determined using DFT calculations. All enthalpic reactions and Gibbs free energies were identified as endothermic and nonspontaneous, respectively. This observation supports the chloride dissociation using 532 nm light. This Rh(III)-BODIPY complex expands the class of visible light-activated Rh(III) photocisplatin analogs that may have potential photodynamic therapeutic activity for the treatment of cancers in hypoxic conditions.

In Situ Ellipsometry Measurements on the Halide Phase Segregation of Mixed Halide Lead Perovskites.

Methylammonium lead iodide bromides MAPb(Brx I1-x )3 are a class of mixed halide lead perovskites, materials that offer high-power conversion efficiencies and bandgap tunability. For these reasons, they are a promising absorber material for future solar cells, although their use is still limited due to several factors. The reversible phase segregation under even low light intensities is one of them, lowering the effective bandgap due to local segregation into iodide-rich and bromide-rich phases. While several studies have been done to illuminate the mechanism and suppression of phase segregation, challenges remain to understand its kinetics. We obtained dynamic ellipsometric measurements from x=0.5 mixed halide lead perovskite thin films protected by a polystyrene layer under green laser light with a power density of ∼11 W/cm2 . Time constants between 1.7(±0.7)×10-3  s-1 for the segregation and 1.5(±0.6)×10-4  s-1 for recovery were calculated. The phase segregation rate constants are surprisingly two orders of magnitude slower than and the recovery rate is consistent with those measured using photoluminescence methods under similar conditions. These results confirm a concern in the literature about the complexity in the phase separation kinetics measured from photoluminescence. We expect ellipsometry to serve as a complementary technique to other spectroscopies in studying mixed-halide lead perovskites phase segregation in the future.

Polymer-solvent interaction and conformational changes at a molecular level: Implication to solvent-assisted deformation and aggregation at the polymer surface.

HYPOTHESIS: We hypothesize that varying the chemical structure of the monomeric unit in a polymer will affect the surface structure and interfacial molecular group orientations of the polymer film leveraging its response to solvents of different chemical affinities. EXPERIMENTS: Poly (2-methoxy ethyl methacrylate) and poly (2-tertbutoxy ethyl methacrylate) thin films exposed to either deuterated water (D2O) or deuterated chloroform (CDCl3) were studied by sum frequency generation (SFG) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM) at the polymer-solvent interface, supported with molecular simulation studies. FINDINGS: SFG spectral analysis of the polymer thin films corroborated molecular re-organization at the surface when exposed to different chemical environments. The AFM height images of the polymer surfaces were homogeneously flat under CDCl3 and showed swollen regions under D2O. Following the removal of D2O, the exposed areas have imprinted, recessed locations and exposure to CDCl3 resulted in the formation of aggregates. The chemical affinity and characteristics of the solvents played a role in conformational change at the polymer surface. It had direct implications to interfacial processes involving adsorption, permeation which eventually leads to swelling, deformation or aggregation, and possibly dissolution.

Direct Observation of Adsorption Morphologies of Cationic Surfactants at the Gold Metal-Liquid Interface.

Understanding interfacial phenomena is important in processes like corrosion, catalysis, and electrochemical reactions. Specifically, in corrosion inhibition, the assembly of adsorbed surfactants at metal-water interfaces in well-packed, ordered layers is desired. We provide direct evidence of the role of alkyl tails of surfactants in the formation of ordered adsorbed layers at metal-water interfaces. We have employed surface-specific sum frequency generation (SFG) spectroscopy to probe the in situ adsorption and self-assembly of cationic surfactants, alkyldimethylbenzyl ammonium bromides of tail lengths n = 4 (C4) and 12 (C12), without any applied potential or stimulus, at the gold-water interface. Our SFG measurements show that C12 Quat adsorbs as an ordered monolayer, whereas C4 Quat adsorbs in a disordered monolayer. All-atom molecular dynamics (MD) simulations of these surfactants corroborate with SFG results. These findings provide new insights on how hydrophobic interactions between alkyl tails of surfactants affect their self-assembly at metal-water interfaces.

Evident phase separation and surface segregation of hydrophobic moieties at the copolymer surface using atomic force microscopy and SFG spectroscopy.

HYPOTHESIS: Copolymers are developed to enhance the overall physical and chemical properties of polymers. The surface nature of a copolymer is relevant to creating efficient materials to improve adhesion and biocompatibility. We hypothesize that the improved adhesion, as a surface property, is due to phase separation, surface segregation, and the overall molecular organization of different polymer components at the copolymer surface. EXPERIMENTS: The surface structure of a copolymer composed of 2-hydroxyethyl methacrylate (HEMA) monomer and 2-phenoxyethyl methacrylate (PhEMA) monomer was analyzed in comparison to the polyHEMA and polyPhEMA homopolymers using atomic force microscopy (AFM) and sum frequency generation (SFG) spectroscopy. FINDINGS: The contrast in the phase images was due to the variance in the hydrophobic level provided by the hydroxyl and phenoxy modified monomers in the copolymer. The distribution of the adhesion values, supporting the presence of hydrophobic moieties, across the polymer surface defined the surface segregation of these two components. SFG spectra of the copolymer thin film showed combined spectral features of both polyHEMA and polyPhEMA thin films at the polymer surface. The tilt angles of the alpha-methyl group of homopolymers using the polarization intensity ratio analysis and the polarization mapping method were estimated to be in the range from 48° to 66°.

Role of the cationic headgroup to conformational changes undergone by shorter alkyl chain surfactant and water molecules at the air-liquid interface.

HYPOTHESIS: Surfactants are commonly used as corrosion inhibitors for oil-and-gas pipelines. The alkyl chain of surfactants and their overall conformation contributes to the adsorption, flotation, and foam separation in the inhibition process. We hypothesize that the conformation of shorter alkyl chains and chemical nature of surfactants has an effect on the ordering of water molecules at the air-water interface which is not yet well understood. EXPERIMENTS: Alkyl (C4, C6, C8, C10, and C12) dimethylbenzylammonium bromides (Quats) were synthesized. Aqueous solutions at 0% and with different salt concentrations were studied at the air-liquid interface using sum frequency generation spectroscopy. Surface tension and pH measurement were also conducted for comparison. FINDINGS: Surfactant solutions at 0%, 1%, and 10% salt showed a zigzag trend for the number of gauche defects. At 0% salt, an increasing trend of OH band intensity at 3182 cm-1 was observed from C6 to C12 SFG spectra. Yet, C4 showed a more prominent SFG signal from strongly hydrogen-bonded water molecules compared to C6. The headgroup's chemical nature was found to play a role in the ordering of water molecules for a C4 alkyl chain length. The OH band intensity decreases with increasing ionic strength.

Insights on the Molecular Characteristics of Molecularly Imprinted Polymers as Monitored by Sum Frequency Generation Spectroscopy.

Sensing in molecularly imprinted polymers (MIPs) requires specific interactions of the imprinted polymer and the approaching template molecule. These interactions are affected by the morphology of the polymer surface, the affinity of the template molecule to the polymer network, and the steric approach. In this particular study, a template molecule, metronidazole, is studied with respect to the typically used methacrylic acid-based imprinted polymer using a combination of bulk and surface techniques. The resulting infrared (IR) spectra exhibited the presence of the template molecule in the polymer matrix as well as their efficient removal after washing. Dipping of the MIP according to what is expected of facile sensing in an aqueous solution of metronidazole did not show any presence of the template molecule in the bulk of the MIP, as observed by IR spectroscopy. However, using sum frequency generation (SFG) spectroscopy, the CH aromatic stretch of the imidazole ring positioned at ∼3100 cm-1 was observed at the polymer surface, including its inner pores or cavities, and at the buried polymer-fused silica interface after dipping. SFG studies have also shown the vibrational signatures of the polymer matrix, the presence of the template molecule on the surface, and the detection of residual template molecules after washing. Increasing the washing time to 50 min has proven to be less effective than increasing the washing cycles to three. However, after the third cycle, reorganization of the polymer matrix was evident as also the complete removal of the template molecule. The observed changes from the acquired images using scanning electron microscopy and atomic force microscopy show the structural morphologies of MIPs and a good distribution of the pores across the MIP surface. The study demonstrates the importance of combining both bulk and surface characterization in providing insight into the template molecule-polymer network interactions.

Orientational Analysis of Monolayers at Low Surface Concentrations Due to an Increased Signal-to-Noise Ratio (S/N) Using Broadband Sum Frequency Generation Vibrational Spectroscopy.

Sum frequency generation (SFG) * Equal contributors. spectroscopy was used to deduce the orientation of the terminal methyl (CH3) group of self-assembled monolayers (SAMs) at the air-solid and air-liquid interfaces at surface concentrations as low as 1% protonated molecules in the presence of 99% deuterated molecules. The SFG spectra of octadecanethiol (ODT) and deuterated octadecanethiol (d37 ODT) SAMs on gold were used for analysis at the air-solid interface. However, the eicosanoic acid (EA) and deuterated EA (d39 EA) SAMs on the water were analyzed at the air-liquid interface. The tilt angle of the terminal CH3 group was estimated to be ∼39 ° for a SAM of 1% ODT : 99% d37 ODT, whereas the tilt angle of the terminal CH3 group of the 1% EA : 99% d39 EA monolayer was estimated to be ∼32 °. The reliability of the orientational analysis at low concentrations was validated by testing the sensitivity of the SFG spectroscopy. A signal-to-noise (S/N) ratio of ∼60 and ∼45 was obtained for the CH3 symmetric stretch (SS) of 1% ODT : 99% d37 ODT and 1% EA : 99% d39 EA, respectively. The estimated increase in S/N ratio values, as a measure of the sensitivity of the SFG spectroscopy, verified the capacity to acquire the SFG spectra at low concentrations of interfacial molecules under ambient conditions. Overall, the orientational analysis of CH3 SS vibrational mode was feasible at low concentrations of protonated molecules due to increased S/N ratio. In support, the improved S/N ratio on varying incident power density of the visible beam was also experimentally demonstrated.

Ultrafast Transient Absorption Spectra of Photoexcited YOYO-1 molecules call for additional investigations of their fluorescence quenching mechanism.

In this report, we observed that YOYO-1 immobilized on a glass surface is much brighter when dried (quantum yield 16±4% in the ambient air) or in hexane than in water (quantum yield ~%).YOYO-1 is a typical cyanine dye that has a photo-isomerization reaction upon light illumination. In order to understand this quenching mechanism, we use femtosecond transient absorption spectroscopy to measure YOYO-1's electron dynamics after excitation directly. By deconvoluting the hot-ground-state absorption and the stimulated emission, the dynamics of electronic relaxation and balance are revealed. The results support the intermolecular charge transfer mechanism better than the intramolecular relaxation mechanism that has been widely believed before. We believe that the first step of the relaxation involves a Dexter charge transfer between the photo-excited YOYO-1 molecule and another guest molecule that is directly bound to the YOYO-1 giving two radicals with opposite signs of charges. The charges are recombined either directly between these two molecules, or both molecules start to rotate and separate from each other. Eventually, the two charges recombined non-radiatively via various pathways. These pathways are reflected on the complicated multi-exponential decay curves of YOYO-1 fluorescence lifetime measurements. This charge transfer mechanism suggests that (1) electrical insulation may help improve the quantum yield of YOYO-1 in polar solutions significantly and (2) a steric hindrance for the intramolecular rotation may have a less significant effect.

Successive Surface Reactions on Hydrophilic Silica for Modified Magnetic Nanoparticle Attachment Probed by Sum-Frequency Generation Spectroscopy.

Successive surface reactions on hydrophilic silica substrates were designed and performed to immobilize ethanolamine-modified magnetic ferrite-based nanoparticle (NP) for surface characterization. The various surfaces were monitored using sum-frequency generation (SFG) spectroscopy. The surface of the hydrophilic quartz substrate was first converted to a vinyl-terminated surface by utilizing a silanization reaction, and then, the surface functional groups were converted to carboxylic-terminated groups via a thiol-ene reaction. The appearance and disappearance of the vinyl (═CH2) peak at ∼2990 cm-1 in the SFG spectra were examined to confirm the success of the silanization and thiol-ene reactions, respectively. Acyl chloride (-COCl) formation from carboxy (-COOH) functional group was then performed for further attachment of magnetic amine-functionalized magnesium ferrite nanoparticles (NPs) via amide bond formation. The scattered NPs attached on the modified silica substrate was then used to study the changes in the spectral profile of the ethanolamine modifier of the NPs for in situ lead(II) (Pb2+) adsorption at the solid-liquid interface using SFG spectroscopy. However, due to the limited number of NPs attached and sensitivity of SFG spectroscopy toward expected change in the modifier spectroscopically, no significant change was observed in the SFG spectrum of the modified silica with magnetic NPs during exposure to Pb2+ solution. Nevertheless, SFG spectroscopy as a surface technique successfully monitored the modifications from a clean fused substrate to -COCl formation that was used to immobilize the decorated magnetic nanoparticles. The method developed in this study can provide a reference for many surface or interfacial studies important for selective attachment of adsorbed organic or inorganic materials or particles.

Sum frequency generation vibrational spectroscopy of methacrylate-based functional monomers at the hydrophilic solid-liquid interface.

2-Substituted ethyl methacrylate monomers were characterized using sum frequency generation vibrational spectroscopy (SFGVS) to study the effect of substituent groups in the organization of the monomers at the monomer-hydrophilic quartz interface. The SFG spectra of the methacrylate-based functional monomers collected at the hydrophilic quartz interface were found to be different from those collected at the air-monomer interface. The various spectral profiles indicated different conformations of the molecular groups at the solid-liquid interface. Moreover, a peak shift of the methyl symmetric stretch from ∼2900 cm-1 to ∼2880 cm-1 was observed from the air-liquid to solid-liquid interface, respectively. This observation of peak shifting is due to two factors: (1) a change in the chemical environment of the methacrylate-based monomer from air-to-liquid surfaces and (2) interaction between the ester carbonyl group of the monomer and the surface hydroxyl silanol group of the amorphous quartz surface. Also, the monomers were characterized in the carbonyl region, which showed the presence of the C[double bond, length as m-dash]O stretch in the SFG spectrum. This result is indicative of the hydrogen bonding interactions existing between the carbonyl group of the monomers and the Si-OH groups of the hydrophilic quartz interface. In addition, the changes in the SFG intensity of the C[double bond, length as m-dash]O peak at ∼1730 cm-1 revealed that the conformation of the C[double bond, length as m-dash]O groups is affected by bulky substituents. Furthermore, the conformational changes of these functionalized monomers at the hydrophilic solid-neat liquid interface was investigated using SFGVS.

Methyl groups of trimethylamine N-oxide orient away from hydrophobic interfaces.

The molecular orientation of trimethylamine N-oxide (TMAO), a powerful protein stabilizer, was explored at aqueous/hydrophobic interfaces using vibrational sum frequency spectroscopy (VSFS). The systems studied included the octadecyltrichlorosilane (OTS)/water interface, which represents an aqueous solution in direct contact with a hydrophobic medium. Surprisingly, the measurements revealed that the methyl groups of TMAO pointed into the aqueous phase and away from the OTS. This orientation may arise from the more hydrophilic nature of methyl groups attached to a strongly electron-withdrawing atom such as a quaternary nitrogen. Additional studies were performed at the air/water interface. This interface showed a high degree of TMAO alignment, but the dangling OH from water was present even at 5 M TAMO. Moreover, the addition of this osmolyte modestly increased the surface tension of the interface. This meant that this species was somewhat depleted at the interface compared to the bulk solution. These findings may have implications for the stabilizing effect of TMAO on proteins. Specifically, the strong hydration required for the methyl groups as well as the oxide moiety should be responsible for the osmolyte's depletion from hydrophobic/aqueous interfaces. Such depletion effects should help stabilize proteins in their folded and native conformations on entropic grounds, although orientational effects may play an additional role.

Chemical imaging of corrosion: sum frequency generation imaging microscopy of cyanide on gold at the solid-liquid interface.

Sum frequency generation (SFG) imaging is used to monitor, in situ, the reaction of cyanide ions with gold surface. Spatial and chemical variations across the surface are observed as a function of time. The initial period resulted in the formation of linearly bound cyanide to gold, and continuous exposure of gold film to cyanide solution led to the presence of higher-coordinated gold-cyanide complexes. These species were identified by their specific position in the SFG vibrational spectrum (2105, 2140, 2170, and 2225 cm(-1)). The relative amounts of these gold-cyanide species varied across the surface as resolved by SFG microscopy.

Nanoscale friction varied by isotopic shifting of surface vibrational frequencies.

Friction converts kinetic energy at sliding interfaces into lattice vibrations, but the detailed mechanisms of this process remain unresolved. Atomic force microscopy measurements reveal that changing the mass of the terminating atoms on a surface, and thus their vibrational frequencies, affects nanoscale friction substantially. We compared hydrogen- and deuterium-terminated single-crystal diamond and silicon surfaces, and in all cases the hydrogenated surface exhibited higher friction. This result implies that the lower natural frequency of chemisorbed deuterium reduces the rate at which the tip's kinetic energy is dissipated. This discovery is consistent with a model describing energy transfer to adsorbates from a moving surface.

Sum frequency generation imaging microscopy of CO on platinum.

Sum frequency vibrational spectroscopy is utilized as an imaging technique to distinguish and compare the local response of carbon monoxide (CO) covered platinum (Pt) polycrystalline surface versus the average response of the investigated area. The Pt electrode was prepared using the standard method and was exposed to approximately 1 atm of CO(g). SFG images and vibrational spectra were obtained where the contrast is based on the intrinsic nature of each peak in the CO vibrational spectrum. The illustration of the images and the chemical maps of CO on the platinum surface showed the distribution of the CO across the observed area. The results obtained by comparing the local and the average response confirmed the spatial distributions of the CO on the platinum sample which are due to several reasons such as dipole-dipole coupling and surface coverage. This finding has a significant contribution toward recognizing that surfaces usually considered homogeneous may in fact be quite heterogeneous.

Sum frequency generation microscopy of microcontact-printed mixed self-assembled monolayers.

Sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The contrast in the images is based on the vibrational spectrum of each component in the monolayer. Mixtures of C16 thiols on gold with CH3 and phenyl termination are imaged with a resolution of approximately 10 microm. Microcontact printing produces films that are different compared to the immersion procedure of forming self-assembled monolayers. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has significant mixing with the molecules deposited from the backfilling solution.