Ultrasensitive Photothermal Spectroscopy: Harnessing the Seebeck Effect for Attogram-Level Detection.

Molecular-level spectroscopy is crucial for sensing and imaging applications, yet detecting and quantifying minuscule quantities of chemicals remain a challenge, especially when they surface adsorb in low numbers. Here, we introduce a photothermal spectroscopic technique that enables the high selectivity sensing of adsorbates with an attogram detection limit. Our approach utilizes the Seebeck effect in a microfabricated nanoscale thermocouple junction, incorporated into the apex of a microcantilever. We observe minimal thermal mass exhibited by the sensor, which maintains exceptional thermal insulation. The temperature variation driving the thermoelectric junction arises from the nonradiative decay of molecular adsorbates' vibrational states on the tip. We demonstrate the detection of photothermal spectra of physisorbed trinitrotoluene (TNT) and dimethyl methylphosphonate (DMMP) molecules, as well as representative polymers, with an estimated mass of 10-18 g.

Enhanced nanoplasmonic heating in standoff sensing of explosive residues with infrared reflection-absorption spectroscopy.

Various standoff sensing techniques employing optical spectroscopy have been developed to address challenges in safely identifying trace amounts of explosives at a distance. A flexible anodic aluminum oxide (AAO) microcantilever and a high-power quantum cascade laser utilized as the infrared (IR) source are used for standoff IR reflection-absorption spectroscopy to detect explosive residues on a metal surface. Standoff sensing of trinitrotoluene (TNT) is demonstrated by exploiting the high thermomechanical sensitivity of a bimetallic AAO microcantilever. Moreover, sputtering gold onto the fabricated AAO nanowells generates a strong scattering and absorption of IR light in the wavelength range of 5.18 µm to 5.85 µm resulting in enhanced nanoplasmonic heating. Utilizing the IR absorption enhancement in this wavelength range, the plasmonic AAO cantilever could detect TNT molecules 7 times better than could the bimetallic AAO cantilever.

Effect of Surface and Interfacial Tension on the Resonance Frequency of Microfluidic Channel Cantilever.

The bending resonance of micro-sized resonators has been utilized to study adsorption of analyte molecules in complex fluids of picogram quantity. Traditionally, the analysis to characterize the resonance frequency has focused solely on the mass change, whereas the effect of interfacial tension of the fluid has been largely neglected. By observing forced vibrations of a microfluidic cantilever filled with a series of alkanes using a laser Doppler vibrometer (LDV), we studied the effect of surface and interfacial tension on the resonance frequency. Here, we incorporated the Young-Laplace equation into the Euler-Bernoulli beam theory to consider extra stress that surface and interface tension exerts on the vibration of the cantilever. Based on the hypothesis that the near-surface region of a continuum is subject to the extra stress, thin surface and interface layers are introduced to our model. The thin layer is subject to an axial force exerted by the extra stress, which in turn affects the transverse vibration of the cantilever. We tested the analytical model by varying the interfacial tension between the silicon nitride microchannel cantilever and the filled alkanes, whose interfacial tension varies with chain length. Compared with the conventional Euler-Bernoulli model, our enhanced model provides a better agreement to the experimental results, shedding light on precision measurements using micro-sized cantilever resonators.

Sample Preparation in Centrifugal Microfluidic Discs for Human Serum Metabolite Analysis by Surface Assisted Laser Desorption/Ionization Mass Spectrometry.

We introduce a centrifugal microfluidic disc that accepts a small volume in (∼5 µL), performs sample cleanup on human serum samples, and delivers a small volume out, for subsequent metabolite analysis by surface assisted laser desorption/ionization (SALDI) mass spectrometry (MS) or hydrophilic interaction liquid chromatography (HILIC)-MS. The centrifugal microfluidic disc improves the MS results by removing proteins and lipids from serum. In the case of SALDI-MS, sample background electrolytes are segregated from analytes during the spotting process by the action of the SALDI-chip during drying, for further cleanup, while HILIC separates the salts in HILIC-MS. The resulting mass spectra of disc-prepared samples show a clean background and high signal-to-noise ratio for metabolite peaks. Several representative ionic metabolites from human serum samples were successfully quantified. The performances of the sample preparation disc for SALDI-MS and HILIC-MS were assessed and were comparable. Reproducibility, sample bias, and detection limits for SALDI-MS compared well to ultrafiltration sample preparation.

Spin photonic forces in non-reciprocal waveguides.

Optical forces acting on particles - controlled by the intensity, polarization and direction of optical beams - have become an important tool in manipulation, sorting and analysis of nano/micro-particles. The nature of these forces has been well understood in reciprocal structures exhibiting time-reversal symmetries. Here, we investigate the nature of optical forces in non-reciprocal structures with non-degenerate counter-propagating modes. We consider the specific case of non-reciprocity induced via translational motion and show that the two counter-propagating modes in a moving slab-waveguide are not degenerate which results in a non-zero lateral and longitudinal force on a nanoparticle. We prove that these anomalous forces are fundamentally connected to near-field photonic spin in optical waveguides and explain their directionality using universal spin-momentum locking of evanescent waves. The presented results show that the interplay of photon spin and non-reciprocity can lead to unique avenues of controlling nanoscale optical forces on-chip.

Injectable Self-Healing Zwitterionic Hydrogels Based on Dynamic Benzoxaborole-Sugar Interactions with Tunable Mechanical Properties.

Dynamic hydrogels based on arylboronic esters have been considered as ideal platforms for biomedical applications given their self-healing and injectable characteristics. However, there still exist some critical issues that need to be addressed or improved, including hydrogel biocompatibility, physiological usability, and tunability of mechanical properties. Here, two kinds of phospholipid bioinspired MPC copolymers, one is zwitterionic copolymer (PMB) containing a fixed 15 mol % of benzoxaborole (pKa ≈ 7.2) groups and the other is zwitterionic glycopolymers (PMG) with varied ratios of sugar groups (20%, 50%, 80%), were synthesized respectively via one-pot facile reversible addition-fragmentation chain transfer (RAFT) polymerization. PMBG hydrogels were formed spontaneously after mixing 10 wt % of PMB and PMG copolymer solutions because of dynamic benzoxaborole-sugar interactions. The mechanical properties of nine hydrogels (3 × 3) with different sugar contents and pHs (7.4, 8.4, 9.4) were carefully studied by rheological measurements, and hydrogels with higher sugar content and higher pH were found to have higher strength. Moreover, similar to other arylboronic ester-based hydrogels, PMBG hydrogels possessed not only self-healing and injectable properties but also pH/sugar responsiveness. Additionally, in vitro cytotoxicity tests of gel extracts on both normal and cancer cells further confirmed the excellent biocompatibility of the hydrogels, which should be ascribed to the biomimetic nature of phosphorylcholine (PC) and sugar residues of the copolymers. Consequently, the zwitterionic dynamic hydrogels provide promising future for diverse biomedical applications.

Flocculation and Dewatering of Mature Fine Tailings Using Temperature-Responsive Cationic Polymers.

Temperature-responsive copolymer with cationic charge was prepared with N-isopropylacrylamide (NIPAm) and 2-aminoethyl methacrylamide hydrochloride (AEMA) by conventional free-radical polymerization. The flocculation performance of the copolymer, poly(AEMA-st-NIPAm), was compared to five different mixture ratios of polyNIPAm and cationic poly(acrylamide-st-diallyldimethylammonium chloride) (poly(AAm-st-DADMAC)). The effects of polymer mixture ratios, polymer dosages, and temperature on solid-liquid separation as a function of initial settling rates (ISR), supernatant turbidity, sediment solid content, and water recovery were investigated. Poly(NIPAm) can facilitate particles aggregation by bridging and hydrogen bonding under lower critical solution temperature (LCST); whereas, at temperature above LCST, the adsorption of poly(NIPAm) chains on particles can be enhanced by hydrophobic interaction. A two-step (25 °C → 50 °C → 25 °C) consolidation can further enhance the sediment solid content by polyNIPAm. While the neutral property of polyNIPAm resulted in high turbidity of supernatant, mixing with poly(AAm-st-DADMAC) increases the clarity of supernatant by neutralization of fine particles. The copolymer poly(AEMA-st-NIPAm) functions as a polyelectrolyte to enhance the polymer adsorption onto particles via electrostatic interactions, thus further improving ISR and supernatant clarity.

Abiotic streamers in a microfluidic system.

In this work, we report the phenomenon of formation of particle aggregates in the form of thin slender strings when a polyacrylamide (PAM) solution, laden with polystyrene (PS) beads is introduced into a microfluidic device containing an array of micropillars. PAM and a dilute solution of PS beads are introduced into the microfluidic channel through two separate inlets and localized particle aggregation is found to occur under certain flow regimes. The particle aggregates initially have a string-like morphology and are tethered at their ends to the micropillar walls, while the structure remains suspended in the fluid medium. Such a morphology inspired us to name these structures streamers. The flow regimes under which streamer formation is observed are quantified through state diagrams. We discuss the streamer formation time-scales and also show that streamer formation is likely the result of the flocculation of PS beads. Streamer formation has implications in investigating particle-laden complex flows through porous media.

Hollow Microtube Resonators via Silicon Self-Assembly toward Subattogram Mass Sensing Applications.

Fluidic resonators with integrated microchannels (hollow resonators) are attractive for mass, density, and volume measurements of single micro/nanoparticles and cells, yet their widespread use is limited by the complexity of their fabrication. Here we report a simple and cost-effective approach for fabricating hollow microtube resonators. A prestructured silicon wafer is annealed at high temperature under a controlled atmosphere to form self-assembled buried cavities. The interiors of these cavities are oxidized to produce thin oxide tubes, following which the surrounding silicon material is selectively etched away to suspend the oxide tubes. This simple three-step process easily produces hollow microtube resonators. We report another innovation in the capping glass wafer where we integrate fluidic access channels and getter materials along with residual gas suction channels. Combined together, only five photolithographic steps and one bonding step are required to fabricate vacuum-packaged hollow microtube resonators that exhibit quality factors as high as ∼ 13,000. We take one step further to explore additionally attractive features including the ability to tune the device responsivity, changing the resonator material, and scaling down the resonator size. The resonator wall thickness of ∼ 120 nm and the channel hydraulic diameter of ∼ 60 nm are demonstrated solely by conventional microfabrication approaches. The unique characteristics of this new fabrication process facilitate the widespread use of hollow microtube resonators, their translation between diverse research fields, and the production of commercially viable devices.

Standoff Mechanical Resonance Spectroscopy Based on Infrared-Sensitive Hydrogel Microcantilevers.

This paper reports a highly sensitive and selective remote chemical sensing platform for surface-adsorbed trace chemicals by using infrared (IR)-sensitive hydrogel microcantilevers. Poly(ethylene glycol) diacrylate (PEG-DA) hydrogel microcantilevers are fabricated by ultraviolet (UV) curing of PEG-DA prepolymer introduced into a poly(dimethylsiloxane) mold. The resonance frequency of a PEG-DA microcantilever exhibits high thermal sensitivity due to IR irradiation/absorption. When a tunable IR laser beam is reflected off a surface coated with target chemical onto a PEG-DA microcantilever, the resonance frequency of the cantilever shifts in proportion to the chemical nature of the target molecules. Dynamic responses of the PEG-DA microcantilever can be obtained in a range of IR wavelengths using a tunable quantum cascade laser that can form the basis for the standoff mechanical resonance spectroscopy (SMRS). Using this SMRS technique, we have selectively detected three compounds, dimethyl methyl phosphonate (DMMP), cyclotrimethylene trinitramine (RDX), and pentaerythritol tetranitrate (PETN), located 4 m away from the PEG-DA microcantilever detector. The experimentally measured limit of detection for PETN trace using the PEG-DA microcantilever was 40 ng/cm2. Overall, the PEG-DA microcantilever is a promising candidate for further exploration and optimization of standoff detection methods.

Strain-induced electrostatic enhancements of BiFeO3 nanowire loops.

Semiconductor nanowires (NWs), due to their intriguing structural and physical properties, offer tremendous potential for future technological applications. The existence of strain in NWs can greatly affect, for instance, their mechanical, electrical and optical properties. Here, we report an extraordinary electrostatic response of semiconductor BiFeO3 NW loops, based on Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM). A substantial ∼300 mV surface potential difference, accompanied by an ∼29% higher surface charge density, was found on the NW loop. We also found that the electrostatic enhancement is strongly related to the strain present at the curvature of the NW loops. We propose that the electric polarization coupled with mechanical strain (piezoelectric effect) or strain gradient (flexoelectricity) as possible reasons to account for our observation. These findings provide new insights into multiferroic based semiconductor NWs under external stimuli as well as significant inspiration towards strain sensors and electromechanical devices with multifunctional sensing abilities.

Photothermal Electrical Resonance Spectroscopy of Physisorbed Molecules on a Nanowire Resonator.

Mid-infrared (IR) photothermal spectroscopy of adsorbed molecules is an ideal technique for molecular recognition in miniature sensors with very small thermal mass. Here, we report on combining the photothermal spectroscopy with electrical resonance of a semiconductor nanowire for enhanced sensitivity, selectivity, and simplified readout. Wide band gap semiconductor bismuth ferrite nanowire, by virtue of its very low thermal mass and abundance of surface states in the band gap, facilitates thermally induced charge carrier trapping in the surface states, which affects its electrical resonance response. Electrical resonance response of the nanowire varies significantly depending on the photothermal spectrum of the adsorbed molecules. We demonstrate highly selective detection of mid-IR photothermal spectral signatures of femtogram level molecules physisorbed on a nanowire by monitoring internal dissipation response at its electrical resonance.

Electronic Nose for Recognition of Volatile Vapor Mixtures Using a Nanopore-Enhanced Opto-Calorimetric Spectroscopy.

An electronic nose (e-nose) for identification and quantification of volatile organic compounds (VOCs) vapor mixtures was developed using nanopore-enhanced opto-calorimetric spectroscopy. Opto-calorimetric spectroscopy based on specific molecular vibrational transitions in the mid infrared (IR) "molecular fingerprint" regime allows highly selective detection of VOCs vapor mixtures. Nanoporous anodic aluminum oxide (AAO) microcantilevers, fabricated using a two-step anodization and simple photolithography process, were utilized as highly sensitive thermomechanical sensors for opto-calorimetric signal transduction. The AAO microcantilevers were optimized by fine-tuning AAO nanopore diameter in order to enhance their thermomechanical sensitivity as well as their surface area. The thermomechanical sensitivity of a bilayer AAO microcantilever with a 60 nm pore diameter was approximately 1 µm/K, which is far superior to that of a bilayer plain silicon (Si) microcantilever. The adsorbed molecules of VOCs mixtures on the AAO microcantilever were fully recognized and quantified by variations of peak positions and amplitudes in the opto-calorimetric IR spectra as well as by shifts in the resonance frequency of the AAO microcantilever with the adsorbed molecules. Furthermore, identification of complex organic compounds with a real industrial sample was demonstrated by this e-nose system.

Mapping and Quantifying Surface Charges on Clay Nanoparticles.

Understanding the electrical properties of clay nanoparticles is very important since they play a crucial role in every aspect of oil sands processing, from bitumen extraction to sedimentation in mature fine tailings (MFT). Here, we report the direct mapping and quantification of surface charges on clay nanoparticles using Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM). The morphology of clean kaolinite clay nanoparticles shows a layered structure, while the corresponding surface potential map shows a layer-dependent charge distribution. More importantly, a surface charge density of 25 nC/cm(2) was estimated for clean kaolinite layers by using EFM measurements. On the other hand, the EFM measurements show that the clay particles obtained from the tailings demonstrate a reduced surface charge density of 7 nC/cm(2), which may be possibly attributed to the presence of various bituminous compounds residing on the clay surfaces.

Determination of charge on asphaltene nanoaggregates in air using electrostatic force microscopy.

In this paper, we provide measurement of charge of asphaltene nanoaggregates in air using electrostatic force microscopy. We obtain the average surface charge density of the nanoaggregates as 43.7 nC/cm(2). Among the different aspects of asphaltene, one of the least known is its charge and the effect of solvent and compositional variability (of asphaltene) in dictating this charge. For aqueous systems, asphaltene charge demonstrates a strong dependence on the pH and the salt concentration, indicating that a possible ionization of the surface groups leads to this charging. On the contrary, for asphaltene in nonpolar media (e.g., toluene and heptane), it is believed that asphaltene native charge is central in dictating this charging. This native charge is the solvent-independent charge or the asphaltene charge in air. Our measurements, therefore, provide the first direct quantification (i.e., a quantification of charge not from the measurement of the asphaltene mobilities, which in turn requires specification of the nonuniform asphaltene size distribution) of this asphaltene native charge by conducting the measurements in air. Similar measurements in a solvent may introduce a solvent-dependent value, thereby forbidding not only the exact quantification of this native charge but also the understanding of the specific role of the solvent. This measurement, therefore, will provide a useful starting point to quantify the mechanism of asphaltene charging in nonpolar solvents with important ramifications in deciphering the role of asphaltene in transport and handling of crude and heavy oils.

Impedimetric detection of pathogenic Gram-positive bacteria using an antimicrobial peptide from class IIa bacteriocins.

Real-time, label-free detection of Gram-positive bacteria with high selectivity and sensitivity is demonstrated using an interdigitated impedimetric array functionalized with naturally produced antimicrobial peptide from class IIa bacteriocins. The antimicrobial peptide, leucocin A, was chemically synthesized and covalently immobilized on interdigitated gold microelectrodes via the interaction between the C-terminal carboxylic acid of the peptide and free amines of a preattached thiolated linker. Exposing the peptide sensor to various concentrations of Gram-positive bacteria generated reproducible impedance spectra that detected peptide-bacteria interactions at a concentration of 1 cell/µL. The peptide sensor also selectively detected Listeria monocytogenes from other Gram-positive strains at a concentration of 10(3) cfu mL(-1). The study highlights that short peptide ligands from bacteriocin class offer high selectivity in bacterial detection and can be used in developing a robust, portable biosensor device to efficiently detect pathogenic Gram-positive bacteria in food samples.

Direct detection and speciation of trace explosives using a nanoporous multifunctional microcantilever.

We have developed a highly selective and sensitive nanomechanical infrared (IR) calorimetric spectrometer for use in the direct detection of ultralow concentrations of explosive vapors using a nanoporous TiO2 cantilever. These cantilevers were fabricated using a two-step anodization and photolithography process. By patterning nanoscale wells onto a cantilever, its surface area is increased by 2 orders of magnitude and the surface is converted into a preconcentrator. Resonant excitation of adsorbed molecules using IR radiation causes the cantilever to bend due to temperature changes originating from the nonradiative decay process. The porous structure of the cantilever increases its thermomechanical sensitivity as well as the number of adsorbed molecules. The system performance was demonstrated by detecting binary explosive mixtures under ambient conditions. The TiO2 sensor surface also allows regeneration through the photocatalytic decomposition of adsorbates under UV irradiation.

Effect of temperature on morphologies of evaporation-triggered asphaltene nanoaggregates.

We use atomic force microscopy to observe the structural changes in petroleum-asphaltene aggregates in air as a function of temperature. The aggregates are obtained by evaporating a toluene solution containing asphaltene. Increase in temperature leads to transition from self-assembled fractal structures to substantially larger mobile "liquid-like" domains that show distinct tendencies of substrate repulsion and self-coalescence. This new aggregation dynamics of asphaltene can be explained by temperature-induced transition of asphaltene from pure amorphous to liquid crystalline phase. Observation of this new phenomenon for asphaltene will have wide implications for asphaltene handling and separation.

Investigation of pH-induced protein conformation changes by nanomechanical deflection.

Broad-spectrum biosensing technologies examine sensor signals using biomarkers, such as proteins, DNA, antibodies, specific cells, and macromolecules, based on direct- or indirect-conformational changes. Here, we have investigated the pH-dependent conformational isomerization of human serum albumin (HSA) using microcantilevers as a sensing platform. Native and denatured proteins were immobilized on cantilever surfaces to understand the effect of pH on conformational changes of the protein with respect to the coupling ligand. Our results show that protonation and deprotonation of amino acid residues on proteins play a significant role in generating charge-induced cantilever deflection. Surface plasmon resonance (SPR) was employed as a complementary technique to validate the results.

Photoacoustic spectroscopy of surface adsorbed molecules using a nanostructured coupled resonator array.

A rapid method of obtaining photoacoustic spectroscopic signals for trace amounts of surface adsorbed molecules using a nanostructured coupled resonator array is described. Explosive molecules adsorbed on a nanoporous anodic aluminum oxide cantilever, which has hexagonally ordered nanowells with diameters and well-to-well distances of 35 nm and 100 nm, respectively, are excited using pulsed infrared (IR) light with a frequency matching the common mode resonance frequency of the coupled resonator. The common mode resonance amplitudes of the coupled resonator as a function of illuminating IR wavelength present a photoacoustic IR absorption spectrum representing the chemical signatures of the adsorbed explosive molecules. In addition, the mass of the adsorbed molecules as an orthogonal signal for quantitative analysis is determined by measuring the variation of the localized, individual mode resonance frequency of a cantilever on the array. The limit of detection of the ternary mixture of explosive molecules (1:1:1 of trinitrotoluene (TNT), cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) is estimated to be ~ 100 ng cm(-2). These multi-modal signals enable us to perform quantitative and rapid chemical sensing and analysis in ambient conditions.