Dual Friction Force/Fluorescence Microscopy.

Friction force microscopy (FFM) is a mode of atomic force microscopy (AFM) that quantifies both normal and horizontal forces against substrates. Recent improvement in its accuracy at nanonewton ranges and the possibility of combining AFM with fluorescence microscopy enabled the simultaneous characterization by FFM and fluorescence microscopy. This Tutorial describes the operation principle of the dual friction force/fluorescence microscopy setup and highlights its emerging applications in mechanochromic materials.

Dual Nanofriction Force Microscopy/Fluorescence Microscopy Imaging Reveals the Enhanced Force Sensitivity of Polydiacetylene by pH and NaCl.

Polydiacetylene (PDA) is a popular mechanochromic material often used in biosensing. The effect of its headgroup-headgroup interactions on thermochromism such as pH or salt concentration dependency has been extensively studied before; however, their effect on mechanochromism at the nanoscale is left unstudied. In this work, nanofriction force microscopy and fluorescence microscopy were combined to study the effect of pH and ionic strength on the polydiacetylene (PDA) force sensitivity at the nanoscale. We found that the increase in pH from 5.7 to 8.2 caused an 8-fold enhancement in force sensitivity. The elevation of NaCl concentration from 10 to 200 mM also made the PDA 5 times more force-sensitive. These results suggest that the PDA force sensitivity at the nanoscale can be conveniently enhanced by "pre-stimulation" with pH or ionic strength.

Mechanical Properties of Polydiacetylene Multilayers Studied by AFM Force Spectroscopy.

The blue-to-red chromatic phase transition of polydiacetylene (PDA) is accompanied by the twist and rearrangement of its side chains, which results in shortening of the conjugation length in the backbone. However, how these morphological changes affect its mechanical properties remains elusive. In this work, force spectroscopy mapping by atomic force microscopy was employed to quantify mechanical parameters of PDA thin films such as breakthrough force and Young's modulus at the monomer, blue, and red phases during the chromatic transition. We found that the breakthrough force increased by 113% and Young's modulus decreased by 21% during the blue-to-red transition, highlighting that the subtle change in the side-chain configuration has a dramatic impact on its mechanical properties.

A novel near-infrared polymethine dye biosensor for rapid and selective detection of lithocholic acid.

Lithocholic acid (LCA), a secondary bile acid, has emerged as a potential early diagnostic biomarker for various liver diseases. In this study, we introduce a novel near-infrared (NIR) polymethine dye-based biosensor, capable of sensitive and selective detection of LCA in phosphate buffer and artificial urine (AU) solutions. The detection mechanism relies on the formation of J-aggregates resulting from the interplay of 3,3-Diethylthiatricarbocyanine iodide (DiSC2(7)) dye molecules and LCA, which induces a distinctive red shift in both absorption and fluorescence spectra. The biosensor demonstrates a detection limit for LCA of 70 µM in PBS solution (pH 7.4), while in AU solution, it responds to an LCA concentration as low as ∼60 µM. Notably, the proposed biosensor exhibits outstanding selectivity for LCA, effectively distinguishing it from common interferents such as uric acid, ascorbic acid, and glucose. This rapid, straightforward, and cost-effective spectrometer-based method underscores its potential for early diagnosis of liver diseases by monitoring LCA concentrations.

Colorimetric response in polydiacetylene at the single domain level using hyperspectral microscopy.

The structural variance of polydiacetylene (PDA) at the nanoscale level, even under the same fabrication conditions, is one of the origins of its poor reproducibility in chemo/biosensing. In this work, we present a spatial map of such structural distributions within a single crystal by taking advantage of the recent development of hyperspectral microscopy at visible wavelengths. Hyperspectral microscopy provides the distribution of absorption spectra at the spatial resolution of standard optical microscopy. By tracking the blue-to-red transition via this technique, we found that heat or pH stimulation leaves a unique pattern in the transition pathways.

Recent progress in polydiacetylene mechanochromism.

Polydiacetylenes (PDAs) are a family of mechanochromic polymers that change color from blue to red and emit fluorescence when exposed to external stimuli, making them extremely popular materials in biosensing. Although several informative reviews on PDA biosensing have been reported in the last few years, their mechanochromism, where external forces induce the color transition, has not been reviewed for a long time. This mini review summarizes recent progress in PDA mechanochromism, with a special focus on the quantitative and nanoscopic data that have emerged in recent years.

Effect of Na2CO3, HF, and CO2 Treatment on the Regeneration of Exhausted Activated Carbon Used in Sintering Flue Gas.

The method of continuous treatment with Na2CO3 solution, HF solution, and CO2 was proposed for the regeneration of the exhausted activated carbon (EAC) produced in the sintering flue gas purification process. In order to obtain the optimal operation conditions, the effect of key parameters such as Na2CO3 solution concentration, HF solution concentration, and CO2 activation temperature on the sulfur conversion rate and regeneration efficiency was analyzed. Also, the N2 adsorption, Brunauer-Emmett-Teller analysis, scanning electron microscopy-energy dispersive spectrometry, X-ray diffraction, X-ray fluorescence, and Fourier transform infrared spectroscopy were adopted to investigate the deactivation reason and the change of the physical-chemical properties. The results showed that the deactivated EAC was mainly due to the deposition of inorganic compounds such as CaSO4, SiO2, and KCl to block the pores. Continuous treatment with Na2CO3 solution and HF solution could remove the inorganic compounds effectively. CO2 activation treatment further developed the blocked porosity and decreased the surface acidity. The optimal conditions for the regeneration of EAC were a Na2CO3 concentration of 0.5 mol/L, an HF concentration of 0.8 mol/L, and a CO2 activation temperature of 1073 K with the activation time of 1 h, corresponding to the specific surface area of 607.91m2/g. In the fourth regeneration cycle, the adsorption performance during the successive adsorption-regeneration process could still maintain a high level and the regeneration efficiency was 95.31%.

A new quinoline-derived highly-sensitive fluorescent probe for the detection of hydrazine with excellent large-emission-shift ratiometric response.

Hydrazine is an important industrial material yet highly toxic and extremely harmful to people's health when over-exposed in the environment, thus monitoring hydrazine is of great significance. In this work, a novel highly fluorescent fluorophore BQ-OH, based on hydroxyl- and benzo[d]oxazole-substituted quinoline structure, was synthesized and esterified with 4-biomobutyric acid to afford a fluorescent probe BQABr for the selective detection of hydrazine. The probe follows SN2(nucleophilic substitution)-cyclization sensing mechanism with remarkable response, excellent sensitivity and selectivity. Spectra experiments in aqueous solutions demonstrated that BQABr exhibited an excellent ratiometric fluorescence response toward hydrazine with two well separated emission bands before/after sensing reaction. Emission peak shifted over 130 nm from 387 nm to 521 nm, and unexpectedly outstanding ratio signal enhancement over 3000-fold was achieved. Due to the large spectra response, a very low detection limit of 5.8 nM (0.19 ppb) was obtained. Selectivity experiment was performed, showing BQABr had nearly no spectra response to other possible disturbing analytes. The probe-coated test papers were fabricated and successfully applied to detect gaseous hydrazine. Furthermore, potential application for the detection of hydrazine in both environmental samples and biological samples (living cells) has been demonstrated.

Soft-Templated Synthesis of Lightweight, Elastic, and Conductive Nanotube Aerogels.

Conductive polymer (CP) nanotubes are fascinating nanostructures with high electrical conductivity, fast charge/discharge capability, and high mechanical strength. Despite these attractive physical properties, progress in the synthesis of CP nanotube hydrogels is still limited. Here, we report a facile and effective approach for the synthesis of polypyrrole (PPy) nanotube hydrogels by using the weakly interconnected network of self-assembled nanotubes of lithocholic acid as a soft template. The PPy nanotube hydrogels are then converted to aerogels by freeze drying, in which PPy nanotubes form elastic and conductive networks with a density of 38 mg/cm3 and an electrical conductivity of 1.13 S/m. The PPy nanotube aerogels are able to sustain a compressive strain as high as 70% and show an excellent cyclic compressibility due to their robust nanotube networks and hierarchically porous structures, which allow the compressive stress to be easily dissipated. Furthermore, PPy nanotube aerogels show negative strain-dependent electrical resistance changes under compressive strains. The lightweight, elastic, and conductive PPy nanotube aerogels may find potential applications in strain sensors, supercapacitors, and tissue scaffolds.