Water exhibits unique interfacial properties that arise from the collective organization of its hydrogen-bond network. Establishing clear links between molecular-scale interactions and macroscopic observables remains a central challenge in understanding the behavior of liquid water. In this work, we combine experimental measurements of the contact angle of sessile water drops with quantum-chemical modeling of small water clusters (H2O)n (n = 2-6) to explore multiscale effects of hydrogen-bond cooperativity. The cluster calculations reveal a nonlinear, saturating evolution of hydrogen-bond geometries with increasing cluster size, reflecting the onset of cooperative many-body effects. Experimentally, the evolution of the apparent contact angle during evaporation is quantified using both conventional geometry and a non-invasive geometrical-optical method based on analysis of the dark refractive ring, which provides independent validation against conventional goniometric measurements. The evaporation dynamics are further interpreted within the diffusion-limited framework of the Popov model, indicating that the temporal evolution of the apparent contact angle is primarily consistent with geometry-controlled mass loss under diffusion-limited conditions, rather than requiring variations in intrinsic surface energy. By combining macroscopic contact-angle measurements with molecular-level cluster analysis, this study offers a qualitative multiscale perspective in which minimal cooperative hydrogen-bond motifs provide molecular context for interpreting interfacial behavior, without implying direct quantitative prediction of macroscopic interfacial observables.

Metal–Organic Frameworks (MOFs) have diverse applications due to their tunable porosity, large surface area, and diverse chemical functionalities. Among them, one of the most researched MOFs is MIL-101(Cr), which, in addition, is very stable in water. We have used a commercially available substance with approximately 300 nm large crystals for the preparation of a sensing nano-thin layer for the emerging water contaminant PFOS, due to its high selectivity towards this compound. Here, we have synthesized 20 nm sized crystals of MIL-101(Cr), which are among the smallest reported, and compared them to the same material with 300 nm sized crystals. The material was characterized by TEM and XPS. It was possible to prepare insoluble monolayers at the air–water interface (Langmuir films), which were characterized with film compression isotherms, Brewster angle microscopy, and surface potential measurements. The Langmuir–Blodgett (LB) method was used to deposit monolayers on Si wafers and 434 MHz Surface Acoustic Wave resonator simultaneously. The LB layers were very stable over time. The smaller-sized MIL-101 (Cr) crystals exhibit denser, more homogeneous water coverage and packing upon compression, with no observable 10–100 µm aggregates. LB monolayers from the 20 nm particles have approximately six times lower surface roughness. The LB monolayer is far from being smooth, but this will allow excellent access to the MOF pores by the tested analyte in a chemical sensing application. The lack of research on depositing presynthesized MOFs using probably the best method for nanoarchitectonics—the LB method—is addressed. The 20 nm sized MOF crystals are the smallest deposited by this method so far.

Abstract Volatile organic compounds (VOCs) are organic chemical compounds found in various products that easily vaporize and reach in the environment under normal conditions. They adversely affect the environment and human health. Hence, portable in-field chemical sensors with the possibility for continuous monitoring of VOCs are in great demand. Such sensors should have a smooth, thin sensing layer for fast detection. After this layer captures the studied VOC, the signal has to be transduced into a proper electrical signal for further manipulation and display. The best method for producing the sensing layer is the Langmuir and Blodgett (LB) method for layer-after-layer deposition. We use LB monolayers from a fluorescently head-labeled phospholipid DPPE-NBD. It forms stable 3D aggregates thus highly increasing the surface-to-volume ratio even when a monolayer is deposited. Therefore, a fast and very sensitive sensor is prepared. We measured VOCs with LB films from lipids with gravimetric detection using a 434 MHz two-port Surface Acoustic Wave (SAW) resonator with gold electrodes. We also used the SAW resonators’ interdigitated electrodes to enhance the signal in the Electrical Impedance Spectroscopy (EIS) detection method. Combining the two complementary transduction techniques – gravimetry and electrical methods on a single device gives additional selectivity to the chemical sensor. So a highly sensitive chemical sensor for VOCs can be prepared with the suggested approach.

We studied the structural and optical properties of organic single monolayers from the phospholipid Dipalmitoyl Phosphatidyl Ethanolamine head-labeled with the fluorescent chromophore Nitrobenzoxadiazole (DPPE-NBD). Nano-thin (∼ 3.1 nm) Langmuir-Blodgett (LB) films were deposited on different solid substrates. The structure of the produced single monolayers was characterized using fluorescence spectroscopy, subnanosecond laser excited Fluorescence Lifetime Imaging Microscopy (FLIM) and Atomic Force Microscopy (AFM). The obtained structural properties of the DPPE-NBD LB single monolayers were used for optimizing their density and the technology of their deposition targeted at chemical sensing applications. At specialized deposition conditions, stable 3D aggregates in the monolayer were formed, thus yielding a high surface-to-volume ratio, which gives high sensitivity and fast response time. The chemical sensitivity was tested towards non-polar and polar Volatile Organic Compounds (VOCs). In this case, the LB deposition was carried on a two-port 434 MHz Surface Acoustic Wave (SAW) resonator. A combination of gravimetry and Electrical Impedance Spectroscopy (EIS) was used for the transduction. Combining two transduction methods on a single device provides additional information for differentiating between compounds with similar responses to one of the methods.

Abstract Blood glucose (BG) concentration is a critical biomarker for the diagnosis and management of diabetes, necessitating the development of efficient detection methods due to the high global mortality associated with the disease. Glucose oxidase (GOx) is commonly employed in fabricating electrochemical BG sensors, with numerous generations of sensors utilizing this enzyme. This study presents a novel approach to preparing a sensing layer using GOx within a Langmuir-Blodgett (LB) film deposition system. Water-soluble GOx molecules, dissolved in the aqueous subphase, exhibit surface activity and generate surface pressure upon adsorption at the air-water interface. Subsequently, a fluorescently labeled phospholipid, DPPE-NBD, is introduced to form an insoluble monolayer (Langmuir film) at the interface. GOx penetrates this layer due to electrostatic and steric interactions, further increasing surface pressure. This nanoscale composite structure is then transferred onto a solid substrate as a monolayer using the LB method. This approach offers two key advantages over conventional deposition techniques: forming a nano-thin, uniform layer—crucial for rapid sensing—and enhanced enzyme protection against environmental degradation. The study reports the successful fabrication and characterization of these GOx-based monolayers, underscoring their potential for application in BG sensing technologies.

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are pollutants of concern due to their long-term persistence in the environment and human health effects. Among them, perfluorooctane sulfonic acid (PFOS) is very ubiquitous and dangerous for health. Currently, the detection levels required by the legislation can be achieved only with expensive laboratory equipment. Hence, there is a need for portable, in-field, and possibly real-time detection. Optical and electrochemical transduction mechanisms are mainly used for the chemical sensors. Here, we report the first gravimetric detection of small-sized molecules like PFOS (MW 500) dissolved in water. A 100 MHz quartz crystal microbalance (QCM) measured at the third harmonic and an even more sensitive 434 MHz two-port surface acoustic wave (SAW) resonator with gold electrodes were used as transducers. The PFOS selective sensing layer was prepared from the metal organic framework (MOF) MIL-101(Cr). Its nano-sized thickness and structure were optimized using the discreet Langmuir–Blodgett (LB) film deposition method. This is the first time that LB multilayers from bulk MOFs have been prepared. The measured frequency downshifts of around 220 kHz per 1 µmol/L of PFOS, a SAW resonator-loaded QL-factor above 2000, and reaction times in the minutes’ range are highly promising for an in-field sensor reaching the water safety directives. Additionally, we use the micrometer-sized interdigitated electrodes of the SAW resonator to strongly enhance the electrochemical impedance spectroscopy (EIS) of the PFOS contamination. Thus, for the first time, we combine the ultra-sensitive gravimetry of small molecules in a water environment with electrical measurements on a single device. This combination provides additional sensor selectivity. Control tests against a bare resonator and two similar compounds prove the concept’s viability. All measurements were performed with pocket-sized tablet-powered devices, thus making the system highly portable and field-deployable. While here we focus on one of the emerging water contaminants, this concept with a different selective coating can be used for other new contaminants.

In this work, a sensitive coating based on Langmuir–Blodgett (LB) films containing monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) with an immobilized glucose oxidase (GOx) enzyme was created. The immobilization of the enzyme in the LB film occurred during the formation of the monolayer. The effect of the immobilization of GOx enzyme molecules on the surface properties of a Langmuir DPPE monolayer was investigated. The sensory properties of the resulting LB DPPE film with an immobilized GOx enzyme in a glucose solution of various concentrations were studied. It has shown that the immobilization of GOx enzyme molecules into the LB DPPE film leads to a rising LB film conductivity with an increasing glucose concentration. Such an effect made it possible to conclude that acoustic methods can be used to determine the concentration of glucose molecules in an aqueous solution. It was found that for an aqueous glucose solution in the concentration range from 0 to 0.8 mg/mL the phase response of the acoustic mode at a frequency of 42.7 MHz has a linear form, and its maximum change is 55°. The maximum change in the insertion loss for this mode was 18 dB for a glucose concentration in the working solution of 0.4 mg/mL. The range of glucose concentrations measured using this method, from 0 to 0.9 mg/mL, corresponds to the corresponding range in the blood. The possibility of changing the conductivity range of a glucose solution depending on the concentration of the GOx enzyme in the LB film will make it possible to develop glucose sensors for higher concentrations. Such technological sensors would be in demand in the food and pharmaceutical industries. The developed technology can become the basis for creating a new generation of acoustoelectronic biosensors in the case of using other enzymatic reactions.

PEO/SNCs based nanocomposite electrolyte membrane complexed with MgBr2 salt (15 wt.%) was fabricated following conventional solution casting technique. The effect of incorporation of salt ions on microstructural properties of host matrix of PEO/SNCs were investigated by means of XRD, FTIR, DSC studies. The modifications in the nature of diffraction peaks and vibrational modes of nanocomposites revealed the formation of strong hydrogen bonding and cross links between SNCs and salt ions. A decrease of melting and glass transition temperatures of host matrix was observed as a result of salt doping. The complex electrochemical impedance measurements were carried out in the applied frequency range of 0.1 Hz – 1 MHz and in the temperature range of 303–373 K. The PEO/SNCs composite complexed with 15 wt.% salt showed room temperature ionic conductivity of 7.8 × 10–8 S/cm promising for soft electronics, solid state ionics and sensing applications.

Properties of the Langmuir-Blodgett (LB) films of arachidic and stearic acids, versus the amount of the films' monolayers were studied and applied for chloroform vapor detection with acoustoelectric high-frequency SAW sensors, based on an AT quartz two-port Rayleigh type SAW resonator (414 MHz) and ST-X quartz SAW delay line (157.5 MHz). Using both devices, it was confirmed that the film with 17 monolayers of stearic acid deposited on the surface of the SAW delay line at a surface pressure of 30 mN/m in the solid phase has the best sensitivity towards chloroform vapors, compared with the same films with other numbers of monolayers. For the SAW resonator sensing using slightly longer arachidic acid molecules, the optimum performance was reached with 17 LB film layers due to a sharper decrease in the Q-factor with mass loading. To understand the background of the result, Atomic Force Microscopy (AFM) in intermittent contact mode was used to study the morphology of the films, depending on the number of monolayers. The presence of the advanced morphology of the film surface with a maximal average roughness (9.3 nm) and surface area (29.7 µm2) was found only for 17-monolayer film. The effects of the chloroform vapors on the amplitude and the phase of the acoustic signal for both SAW devices at 20 °C were measured and compared with those for toluene and ethanol vapors; the largest responses were detected for chloroform vapor. For the film with an optimal number of monolayers, the largest amplitude response was measured for the resonator-based device. Conversely, the largest change in the acoustic phase produced by chloroform adsorption was measured for delay-line configuration. Finally, it was established that the gas responses for both devices coated with the LB films are completely restored 60 s after chamber cleaning with dry air.

Rayleigh surface acoustic wave (RSAW)-based resonant sensors, functionalized with single and multiple monomolecular layers of Langmuir–Blodgett (LB) films, were thickness and density optimized for the detection of volatile organic compounds (VOC), which could impose a serious threat on the environment and human health. Single layers of a phospholipid (SLP), hexane dissolved arachidic acid (HDAA), and chloroform dissolved arachidic acid (CDAA) were used for the LB film preparation. Several layers of these compounds were deposited on top of each other onto the active surface of high-Q 434 MHz two-port RSAW resonators in a LB trough to prepare a highly sensitive vapor detection quartz surface microbalance (QSM). Frequency shift was measured with a vector network analyzer (VNA). These devices were probed with saturated vapors of hexane, chloroform, methanol, acetone, ethanol, and water after each deposited layer to test the behavior of the QSM’s insertion loss, loaded Q, vapor sensitivity, and to find the optimum trade-off between these parameters for the best real-life sensor performance. With 2200 ppm and 3700 ppm sensitivity to chloroform, HDAA and CDAA coated QSM devices reached the optimum sensor performance at 15 and 11–15 monolayers, respectively. Surface pressure optimized single monolayers of phospholipid LB films were found to provide up to 530 ppm sensitivity to chloroform vapors with a negligible reduction in loss and loaded Q. This vapor sensitivity is higher than the mass of the sensing layer itself, making SLP films an excellent choice for QSM functionalization.

Langmuir-Blodgett (LB) monolayer films deposited from a Dipalmytoyl Phosphatidyl Ethanolamine head labelled with Nitrbenzoxadiazole (DPPE-NBD), have been studied for their ability to be used as sensing layer for chemical gas sensors. DPPE-NBD molecules are known for their similarity to bio membrane molecules. Therefore, it is expected that they can serve as a matrix for immobilization of proteins, enzymes, aptamers, while preserving their function for selective reaction with organic analytes. In this study, LB monolayers of DPPE-NBD were deposited on Rayleigh Surface Acoustic Wave (RSAW) resonant devices working at 411 MHz and simultaneously on ultra-flat Si wafer substrates for Atomic Force Microscopy (AFM) inspection. Depositions were carried out both at a low surface pressure where the liquid-expanded phase dominates, and at a high surface pressure, where the liquid-condensed (solid) phase dominates. AFM topography reveals the liquid and solid phase coexistence as well as the formation of 3D pyramids of 3 to 30 nm height and 50 to 1000 nm in diameter. In a vapour sensing experiment, the RSAW devices were exposed subsequently to vapours of 6 volatile organic compounds (VOCs) and water. The most significant resonant frequency shift of 225 kHz which corresponds to 11.42 ng mass change was observed with chloroform vapours when the substrate with the solid phase LB monolayer was used as a sensing layer. Adsorption and desorption of the vapours was very fast (a few seconds) and completely reversible. The higher-pressure deposited LB film demonstrates higher sensitivity to all gases. Mechanisms of this behaviour are discussed.

Methanol gas-sensitive element was designed from 3-nm phospholipid Langmuir-Blodgett (LB) film deposited on interdigitated gold microelectrodes. The LB molecular monolayer was prepared from phospholipid dipalmitoyl-phosphatidyl-ethanolamine (DPPE). In view of gas detection applications, we have studied the electrical impedance of DPPE LB sensor when it was exposed on vapors of methanol at ambient temperature. The response of the DPPE LB sensing film to methanol vapors was tested by complex electrical impedance spectroscopy in the frequency range 0.1 Hz -100 KHz of the applied electric field.

The process of formation of a Langmuir-Schaefer (LS) matrix based on a mixed monolayer of arachidic acid (AA) and 8 nm CdSe/CdS/ZnS quantum dots (QDs) stabilized by molecules of trioctylphosphine oxide (TOPO) was investigated. The change in the morphology, monolayer compressibility, and area per elementary cell of the created mixed monolayers, depending on the ratio of the components, was studied. It is shown that the change in the morphology of Langmuir-Blodgett (LB) monolayers begins to occur at a ratio between the number of QDs and AA molecules of 1:24. Dendrimeric structures with a thickness of the order of 30-40 nm appear in the mixed monolayer when LB film deposition was carried out above the collapse surface pressure of a Langmuir film from only TOPO-covered QDs. Information on the dependence of the morphology of such structures on the molar ratio of the components is necessary for the production of ordered 2D nanostructures containing 0D and 1D objects with quantum bonds. Such nanostructures can be used in nanoelectronic and optoelectronic devices as a sensitive sensor element. The obtained results would be relevant for any type of spherical shape nanoparticles.

Fluorescently labelled phospholipids are promising matrix molecules for the sensing layer in chemical biosensor applications. However, their electrical properties in a well-ordered monolayer deposited on a conducting substrate have not been investigated. Here we combine the powerful methods of Scanning Kelvin Probe Microscopy with nanometer lateral resolution and Electrical Impedance Spectroscopy (EIS) to investigate these properties. For the EIS measurements we use the interdigitated gold electrodes of a Surface Acoustic Wave resonator to measure electrical properties parallel to the substrate, which is a novel approach. Molecules behave as dielectrics with resistance around 440 GΩ. Lower resistance is observed at the boundaries of the micrometer sized solid phase domains where the film is the liquid phase. On exposure to chloroform vapors over two orders decrease in resistance was observed which was completely reversible.

This work addresses experimental study on phospholipid Langmuir-Blodgett (LB) films for detection of vapours of volatile organic compounds such as acetone and methanol, at room temperature. For that purpose, LB molecular monolayers of phospholipid dipalmitoyl-phosphatidyl-ethanolamine (DPPE) were deposited on Surface Acoustic Wave Resonator (SAWR) thus forming a sensor element. To test the suitability of the investigated DPPE LB films on SAWRs for practical gas sensing applications, an electrical impedimetric approach in the frequency range 0.1-1 MHz was applied. As a result of vapour sorption on the film, the impedance response of the DPPE LB films is considerably changed that is proper for detection of acetone and methanol vapours.

Biosensors are a promising field driven by research and industry needs. Large number of applications require highly selective and sensitive fast acting biosensors. We target water quality monitoring, cancer research and drug discovery applications. In the biosensor development crucial is the active bioreceptor layer which interacts with the analyte. Here we propose ultra-thin organic films prepared by the Langmuir-Blodgett method from a fluorescently labelled phospholipid. The average thickness of the layer is 3 nm suggesting fast reaction times. From this layer protrude stable over time around 10 nm high spikes with diameters typically from 50 to 300 nm. Thus a well-developed 3D structure is achieved which should yield increased sensor sensitivity for advanced chemical sensor applications.

Over the years several new effects in fluorescently labelled Langmuir monolayers or Langmuir-Blodgett (LB) films, deposited on solid support plates, have been reported. Mostly layers from Dipalmytoyl Phosphatidyl Ethanolamine head labelled with Nitrbenzoxadiazole (DPPE-NBD) have been studied. This molecule behaves exactly as the DPPE molecule which is part of biological membranes. So it is expected that DPPE-NBD molecules can serve as an appropriate matrix for implementation of selectively reacting proteins or enzymes while preserving their function. This combination can act as an active layer in biosensor applications for operation in either gas or liquid environments and this molecule could further increase the system sensitivity in such applications. Addition of the chromophore NBD head group also stabilizes the film and allows for multilayer deposition which is not possible for phospholipids. In this study, LB monolayers were deposited on glass substrates at different surface pressures and with or without Cadmium ions in the water subphase. They were studied using fluorescence spectroscopy and Atomic Force Microscopy (AFM). In view of possible biosensor applications, LB layer by layer deposition was performed on Rayleigh Surface Acoustic Wave (SAW) resonant devices working at 440 MHz. These can effectively be used to transduce the biosensor signal from the active layer by providing a mass proportional frequency output. Layers from DPPE-NBD that might also be suitable for possible SAW based biosensor applications were deposited at a surface pressure of 10 mN/m. AFM spectroscopy reveals the phase coexistence of liquid and solid phases as well as higher bilayer nanosized cylinders. Fluorescence intensity was strongly self-quenched at this pressure and fluorescence lifetime spectroscopy shows a complex behaviour with a 3-rd order exponential decay. A frequency downshift of 10 KHz per monolayer was measured with the coated SAW device which results in a deposited mass of about 1 ng/monolayer.

Detection of some pollutants in water or some markers in blood require high sensitive techniques. Apart from expensive laboratory equipment, biosensors are another possibility. In the biosensor development crucial is the active bioreceptor layer which interacts with the analyte. Here we propose nano-thin organic films prepared by the Langmuir-Blodgett method from a fluorescently labelled phospholipid with well-developed 3D structure, which should yield increased sensor sensitivity. The average thickness of the layer is 3 nm suggesting fast reaction times. We test this structure by detecting Cd2+ ions dissolved in pure water. The transduction of the signal is performed in two ways: optically, by measuring fluorescence intensity; and with a newly developed setup for electrochemical impedance spectroscopy.

Fluorescently head labelled phospholipids form stable organized organic monolayers at the air-water interface (Langmuir films) and can be deposited on solid supports (glass or Surface Plasmon Resonance (SPR)) substrates as Langmuir-Blodgett (LB) films. In a subsequent work LB mono- and multilayers were deposited on Surface Acoustic Wave (SAW) devices. LB monolayers arc very stable and hard to remove while the multilayers, typically for phospholipids, are difficult to deposit and can be removed more easily. Several new effects were discovered in Dipalmitoyl Phosphatidyl Ethanolamine head labelled with NitroBenzoxaDiazole (DPPE-NBD) molecule in the past which still need detailed explanation. Previous research of the Langmuir film was combined with the present study of fluorescence spectra and decay kinetics of LB monolayers and a 15 layer SPR substrate, which is a plastic substrate with gold evaporated electrodes. Langmuir-Blodgett films were deposited as monolayers or multilayers on glass or gold structured supports at different surface pressures and from either pure water subphase or with presence of Cadmium ions (Cd2+) in the water subphase. Atomic force microscopy reveals phase coexistence of liquid phase, solid phase and bilayer or higher height hundreds of nanometers in diameter cylinders. This rich polymorphism leads to complicated fluorescence lifetime spectra. Understanding of the structure and aggregation in these films can help with their possible future applications.

Single component monolayers from Dipalamitoyl Phosphatidyl Ethanolamine head labelled with the fluorescent chromophore NitroBenzoxaDiazole (DP-NBD-PE) were investigated at the air-water interface as Langmuir films and deposited on silicon wafers or glass plates as Langmuir-Blodgett (LB) films. A step compression and monitoring of the pressure relaxation together with domain formation as observed with Brewster angle microscopy suggests that main transition from liquid expanded to liquid compressed state start at around 6.9 mN/m though on the isotherm it starts around 9 mN/m at 25° C. Brewster angle microscopy also reveals a nonuniform structure in the monolayer. 3D aggregates - cylinders with 50 – 150 nm diameter and bilayer height were observed with Atomic Force Microscopy when deposition was carried at 7 mN/m, above the main phase transition but considerably lower than the equilibrium spreading pressure of 19.6 mN/m for this molecule. When LB film deposition is carried just below the main phase transition a uniform height layer of film in a liquid phase is observed with AFM with no structures. Both compression and deposition were carried at very low speeds with large time to relax in ordered to avoid kinetic effects. These 3D aggregates are not due to the transfer process or interaction with the substrate. These aggregates provide a highly developed area combined with monolayer thick structure which can produce very fast and highly sensitive biosensors.