Arindam Phani

Microcantilever sensors can measure adsorption in two ways. Resonance frequency mainly follows total mass uptake, whereas static bending follows surface stress. These two signals do not always change smoothly together. In many bending measurements, sharp jumps, plateaus, and burst-like features appear on top of a smooth adsorption or recovery curve, and these features are often treated as noise or drift. Here, we show that they can contain useful physical information. We separate the smooth bending response from the residual signal and use detrended fluctuation analysis to measure how persistent the residual activity is through a scaling exponent, α. The experiments show strongly persistent residual fluctuations. A minimal kinetic Ising model, with the Hamiltonian unchanged but the transition rule made locally cooperative, shows that such burst-like activity and persistent scaling can arise from local pathway selection alone. The results reveal a residual route to cooperative surface-stress dynamics and suggest how local transition rules can generate abrupt collective switching.

Transitions often begin through weak precursor dynamics that reorganize a system before the dominant transformation becomes apparent, yet these early signatures can remain distributed and hidden within a single observable. Here, hollow microfluidic cantilevers resolve such onset behavior by tracking the resonance trajectory of picoliter-confined samples during controlled slow heating. Using DNA premelting as a model system, we identify a lower-temperature spectral departure that persists across three duplex sequence backgrounds before the main melting response. Projection of the same spectra into buffer-referenced response coordinates enhances the contrast of this onset-associated change without replacing the primary spectral evidence. The resulting operational marker identifies when weak local fluctuations first become mechanically consequential in the confined fluid system. More broadly, the approach detects the transition onset as a persistent reorganization of a measured trajectory rather than a large scalar signal.

Biological systems achieve parts-per-billion (ppb) sensitivity in gas detection by tracking molecular fluctuations over time─a level of precision that remains difficult to replicate in engineered sensors. Conventional sensing relies on adsorption processes that require activation energies (E_a) ∼10 k_BT, resulting in exponentially long equilibration times and limited selectivity due to small differences in E_a among analytes. Here, we show that volatile organics interacting with a ∼200 nm-thick nanoporous metal-organic framework (MOF), when subjected to shear-induced strain via a quartz crystal microbalance (QCM), exhibit a secondary fluctuational adsorption time scale distinct from the steady-state response. This emergent kinetic signature allows for reliable molecular discrimination at sensitivities down to ∼100 ppb. Our approach introduces a new selectivity metric based on dynamic adsorption kinetics, opening avenues for real-time molecular identification in environmental monitoring, portable diagnostics, and selective detection in chemically complex settings.

Hexavalent chromium (Cr⁶⁺) ions in drinking water pose a significant risk to human health, being a leading cause for neurological disorders, organ damage, and infertility. This study introduces an ultrasensitive method for detecting trace Cr⁶⁺ over a wide concentration range (≈ 100 pM – 100 µM) through fluorescence enhancement signatures via integration of both covalent and non‐covalent interaction strategies on carbon quantum dots (CQD). The covalent functionalization is achieved from dual‐functionalized CQD (CQD‐(NH₂, COOH)) derived from coffee‐waste. Additionally, the covalent and non‐covalent approach integrates CQD‐(NH₂, COOH) with graphitic carbon nitride (g‐C₃N₄) to form a 2D/2D heterostructure. The synergy between CQD‐(NH₂, COOH) and g‐C₃N₄ introduces a mid‐gap band in their band structure, allowing multiple carrier excitation and recombination states, significantly enhancing the fluorescence quenching signal. This combination allows to achieve Cr⁶⁺ detection sensitivity down to ≈100 pM concentration—matching the World Health Organization's 96 pM permissible limit of total Cr in drinking water. Furthermore, a 70 pM detection limit is reported for Cr⁶⁺ in a mixture of twelve ions, including cations and anions, surpassing current state‐of‐the‐art detection limits. These results highlight the potential of dual covalent and non‐covalent modification strategy in nanomaterials to set new standards in ultrasensitive and wide‐range fluorescent sensing applications.

Conventional tapping-mode atomic force microscopy (AFM) is limited in its ability to resolve time-varying interaction forces during rapid and stochastic molecular transitions at a sample pixel while scanning. Resolving these transitions is essential for tracking molecular assembly and rearrangements in real-time. We introduce Transitional-tapping (Tt) AFM, a technique that achieves this by directly measuring sample stochasticity from higher harmonic tip interaction forces. Tt-AFM harmonizes tip-sample motion by fine-tuning tip oscillations to sustain continuous engagement during scanning. We demonstrate its capability by uncovering transient native-like protein intermediates, that align remarkably with AlphaFold and Protein Data Bank predictions. The imaged intermediates retain native folds but undergo subtle chain rearrangements, persisting as ‘rest-stops’ for nanoseconds along the folding landscape to stabilize. Tip dynamics reveal that shear-thinning guides folding through sub-diffusive Lévy-jump kinetics. Transitional-tapping fundamentally transforms AFM’s capability to probe stochasticity and intermediate energetics at sub-k_BT precision, unlocking new frontiers in soft-matter AFM.

Organic photodetectors (OPDs) hold immense promise for optoelectronic applications. Here a zero‐biased, high‐performance organic photodetector employing a 2D organic heterostructure is introduced. The structure combines carbon quantum dots (CQDs) with nitrogen self‐doped graphitic carbon nitride (g‐C₃N₄₊) and is tested for alternating current (AC) photodetection on an interdigitated electrode platform. The study reveals extraordinary performance driven by the synergistic effects of efficient charge excitation, separation, and emission within the 2D/2D CQD/g‐C₃N₄₊ heterostructure, leveraging mechanisms of photoconduction, photogating, and fluorescence. A unique convergence to similar rise and decay times in the order of 2.9 ms is observed at higher frequencies in the visible (Vis) spectrum. Benchmarking against state‐of‐the‐art OPDs shows ultrahigh specific detectivity (4.60 × 10¹⁸ Jones), ultrahigh responsivity (1.43 × 10⁷ A W⁻¹), high external quantum efficiency (43 × 10⁷%) at an optical intensity of 3.56 × 10⁻⁴ mW cm⁻² and a wavelength of 405 nm while delivering competitive performance at 532 and 635 nm as well. Moreover, a large linear dynamic range of 86–162 dB in the Vis spectrum is obtained. These enhancements promise development of a new generation of OPDs to advance light sensing and imaging applications at high frequency, marking a significant milestone in optoelectronic device engineering.

2D metal–organic frameworks (MOFs) offer high surface area and unique accessibility to active adsorption sites making them appealing for gas sensing applications. 2D MOFs‐based sensors are gaining traction for detecting hazardous flu‐gases such as ammonia selectively at low concentrations. Fluorescent and colorimetric sensing are promising techniques offering high sensitivity, selectivity, and rapid response in simple applications. In this work, Zn‐BTC is synthesized as 2D‐MOFs nanosheet with approximate thickness of 2.52 nm via a fast, facile, direct synthesis technique. The introduction of 8‐hydroxyquinoline during synthesis forms fluorescent compounds with zinc (ZnQ) which is encapsulated and decorated onto Zn‐BTC. Inherent charges on ZnQ lead to the agglomeration of multiple 2D‐flakes forming ZnQ@Zn‐BTC multi‐flaked nano‐discs. The synthesized material shows visible color change upon exposure to ammonia from white to ivory. In addition, selective fluorescence quenching is observed under ultraviolet illumination (λ_ex = 365 nm) when ZnQ@Zn‐BTC is exposed to ammonia. The limit of detection reaches 0.27 ppm as a dried film for gaseous sensing and 60.8 nm in liquid phase fluorescence quenching, respectively. The observed high sensitivity and selectivity are attributed to the manipulation of active sites of 2D‐MOFs nanosheet with ZnQ. Functionalization also limits the degradation and breakdown of ZnQ@Zn‐BTC.