We report on the simultaneous monitoring of sweat lactate concentration and sweat secretion rate. For this aim lactate oxidase-Prussian Blue enzyme-nanozyme type lactate biosensors were elaborated. ...The use of siloxane-perfluorosulfonated ionomer composite membrane for enzyme-nanozyme immobilization results in the biosensor displaying flux independence in the whole range of physiological sweat secretion rates (0.025–2 μl cm-2 min-1). On the contrary, current response of the biosensor based on solely siloxane membranes becomes saturated at physiological sweat lactate concentration, depending mostly on the flow rate. Accordingly, for simultaneous monitoring of sweat lactate concentration and its secretion rate both flow-through biosensors were integrated with high-accuracy wearable electronic devices allowing real-time remote monitoring. As found, during exhaustive physical exercise sweat secretion rate and lactate content are independent of each other, thus, confirming that this excretory liquid is suitable for non-invasive diagnostics.
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•Flow-independent enzyme-nanozyme lactate biosensors elaborated.•Wearable high-accuracy device for real-time remote metabolite monitoring designed.•The independence of sweat secretion rate and lactate content shown.
Despite non-invasive instant monitoring of sweat metabolites is becoming a general trend in early diagnostics and screening, the reliability and accuracy of the on-skin electrochemical biosensors in ...real-life scenarios still remain questionable. As a rule, mass transport effects in scantily excreted liquids are ignored, when considering the design of such wearable setups. Here we provide a comprehensive investigation of the disruption factors for commonly used Prussian Blue based (bio)sensors under different hydrodynamic conditions (2 × 10−5 – 5 × 100 mm s−1 electrolyte velocity). A huge effect of flow on the (bio)sensors response has been revealed and explained with transport limitations for both analyte influx and reaction product outflux. It suggests no need for improving the sensor sensitivity, while minimizing analyte consumption and enhancing product withdrawal. Some strategies concerning measurement schemes and sensor design ensuring reliable sweat analysis have been discussed and illustrated for lactate and glucose on-skin monitoring.
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We propose pulse power generation (PPG) amperometry as an advanced readout realized for Prussian blue (PB)-based (bio)sensors. In contrast to the conventional power generation mode, when the current ...response is generated upon continuous short-circuiting, the suggested pulse regime is fulfilled by periodic opening and shorting of the circuit. Despite PB being electroactive, the pulse readout is advantageous over conventional steady-state power generation, providing up to a 15-fold increased signal-to-background ratio as well as dramatically improved sensitivity exceeding 10 A·M–1·cm–2 for H2O2 sensors and 3.9 A·M–1·cm–2 for glucose biosensors. Such analytical performance characteristics are, most probably, achieved due to the enrichment of the diffusion layer by analyte mass transfer from the bulk upon opening of the circuit. Due to an improved sensitivity-to-background ratio, reduced flow-rate dependence, and enhanced operational stability, the regime allows reliable monitoring of blood glucose variations through sweat analysis with the on-skin device.
The paper describes a comparative study of swelling processes in free-standing graphene oxide (GO) membranes and GO laminates encapsulated with epoxy glue. For free-standing graphene oxide membranes, ...a huge variation in d-spacing in the range of 8–12 Å depending on the ambient humidity and from 12 to >30 Å depending on the electrolyte type and its concentration was revealed using direct in situ and in operando XRD studies. Limited swelling at various humidity levels as well as in electrolyte solution with low constriction/expansion of epoxy-encapsulated GO is counterposed to that of free-standing graphene oxides. The swelling suppression was explained by both physical constriction and the intercalation of amines into GO laminates, which was proved by local EDX studies. This results in ion diffusivity variation for over 2 orders of magnitude in free-standing and constrained graphene oxide membranes and provides factual evidence for tunable sieving of ions with confined graphene oxides.
Here we report on the spontaneous assembly of Ti
3
C
2
T
x
MXene flakes into monolayer films at the liquid-air interface. According to X-ray reflectivity and grazing incidence X-ray fluorescence both ...the structure of the layers and assembly kinetics depends on the pH value of the solution. At pH > 4 MXene flakes form a single ∼1.5 nm thick layer carrying a negative charge, while in the acidic medium the layer contains coordinated anions with the formation of the Br
aq
−
/Ti
3
C
2
T
x
/subphase interface. The surface layer compression allows the assembling of MXene flakes into a dense monolayer films with the surface coverage of up to 96% and surface pressure exceeding 40 mN m
−1
in the case of the acidic subphase. The films can readily be transferred onto solid substrates by the conventional Langmuir-Blodgett approach or modified by surfactants to form MXene/surfactant composite films.
Here we report on the spontaneous assembly of Ti
3
C
2
T
x
MXene flakes into monolayer films at the liquid-air interface.
The interest in alternative energy sources grows rapidly and demands improved materials. The cutting-edge investigations focus attention on the development and optimization of solid electrolytes for ...advanced energy storage. Their chemical and structural stability defines both battery performance and lifetime, yet it is studied poorly even for well-known superionic conductors such as NASICON-based compounds. In this work, we studied the Li1.3Al0.3Ti1.7(PO4)3 (LATP) stability toward water. Corresponding ceramics were synthesized in pellet form through the solid-state reaction and had been immersed in deionized water for different periods of time with subsequent electrochemical (electrochemical impedance spectroscopy), structural (powder X-ray diffraction analysis, Raman spectroscopy, computational modeling), chemical (ceramicsenergy-dispersive X-ray spectroscopy; mother-solutionsinductively coupled plasma mass spectrometry), and morphological (scanning and transmission electron microscopy) analyses. Water exposure triggers drastic conductivity losses (64% for σt) with accompanying lithium elution (exceeds 13 atomic%) and unit cell shrinkage. All these changes reach a plateau after 2 h of water exposure.
Structural ordering in the concentrated magnetic colloids containing 50 × 5 nm hard magnetic disc-like SrFe
12
O
19
nanoparticles was investigated by cryogenic scanning electron microscopy, optical ...microscopy, magnetic measurements, and small-angle X-ray scattering. It was revealed that macroscopically homogeneous magnetic liquid consists of dynamic threads of stacked nanoparticles. The threads align into quasiperiodic arrays with the distances between individual threads of a few micrometers. They also can form pseudodomain structures with ~ 90° domain boundaries realized through T-type thread interconnects. The effects of magnetic attraction and electrostatic repulsion on the equilibrium interplatelet distance in the threads were studied. It was demonstrated that this distance can be tuned by the control of the particles charge and electric double layer screening from Stern layer thickness (~ 1 nm) to tens of nanometers. It was shown that the permanent magnetic field is not able to cause any structural changes in the ordered magnetic liquid phase, while alternating field draws particles apart by their vibrations. External variation of interparticle distance up to 6% was achieved using an alternating magnetic field of low intensity. Experimental data were complemented by the theoretical models of screened electrostatic interactions between spherical and platelike magnetic particles. The last model provides good predictive power and correlates with the experimental data. The stabilization energy of the condensed phase in the order of 1–10
k
B
T
was derived from the model. An approach allows controlling of an equilibrium interparticle distance and interparticle distance distribution by adjusting the magnetization and surface charge of the particles as well as the ionic strength of the solvent.
Self-organization of pores in anodic aluminum oxide (AAO) is a unique phenomenon attracting significant interest from both fundamental and practical points of view. Although the influence of ...crystallographic orientation of Al substrate on the degree of pore ordering has been clearly demonstrated, the information about the kinetics of self-organization in AAO formed on various low-index facets of Al is absent. Here, the kinetics of pore ordering during anodizing of low-index surfaces of Al single crystals is studied by using the in situ small-angle X-ray scattering technique. The fastest decrease in the dispersion of pore growth direction is found in the case of anodizing of the Al(100) substrate. To obtain the comparable out-of-plane pore ordering in the case of Al(111) anodizing, the charge density should be doubled. On the contrary, on the Al(111) the fastest kinetics of in-plane orientational pore ordering is observed. The average domain size in the AAO films grown on Al(100) and Al(110) is limited by about 20 lattice periods, whereas the unlimited growth of the domains is observed in the case of the Al(111) substrate. The pore organization into hexagonal domains is found to be much faster than the development of long-range in-plane orientational pore order.
The structural organization of compounds in a confined space of nanometer-scale cavities is of fundamental importance for understanding the basic principles for atomic structure design at the ...nanolevel. Here, we explore size-dependent structure relations between one-dimensional PbTe nanocrystals and carbon nanotube containers in the diameter range of 2.0–1.25 nm using high-resolution transmission electron microscopy and ab initio calculations. Upon decrease of the confining volume, one-dimensional crystals reveal gradual thinning, with the structure being cut from the bulk in either a or a growth direction until a certain limit of ∼1.3 nm. This corresponds to the situation when a stoichiometric (uncharged) crystal does not fit into the cavity dimensions. As a result of the in-tube charge compensation, one-dimensional superstructures with nanometer-scale atomic density modulations are formed by a periodic addition of peripheral extra atoms to the main motif. Structural changes in the crystallographic configuration of the composites entail the redistribution of charge density on single-walled carbon nanotube walls and the possible appearance of the electron density wave. The variation of the potential attains 0.4 eV, corresponding to charge density fluctuations of 0.14 e/atom.
The flow of isobutane and of freon 142b (1-chloro-1,1-difluoro-ethane) through anodic alumina membranes with pore diameters between 18 and 60 nm in a capillary condensation regime is experimentally ...and theoretically explored. The capillary condensation effect increases the membrane permeance for condensable gases from 25 to 150 m
3
(STP) m
−2
bar
−1
h
−1
at certain conditions. To describe the experimental results, a model is suggested accounting for heat transfer from the condensing to the evaporating meniscus, different boundary conditions for the heat transfer between the environment and the membrane, and wettability of the pore wall. The proposed model indicates a large influence of heat supply from the environment to the membrane on the permeance in the capillary condensation regime and a moderate influence of condensate contact angle in the range of 0-60°. Measuring the temperature of the permeate side of the membrane allows to find a suitable boundary condition to describe heat transfer. The obtained boundary condition yields an excellent fit of experimental results of condensate flow through membranes with different pore diameters for the two utilized fluids. Also, confocal Raman spectroscopy gave evidence on the fraction of pores filled with condensate.
The flow of isobutane and of freon 142b (1-chloro-1,1-difluoro-ethane) through anodic alumina membranes with pore diameters between 18 and 60 nm in a capillary condensation regime is experimentally and theoretically explored.
