A new methodology has been demonstrated for ultratrace detection of Hg(2+), working at the limit of a few tens of metal ions. Bright, red luminescent atomically precise gold clusters, Au@BSA (BSA, bovine serum albumin), coated on Nylon-6 nanofibers were used for these measurements. A green emitting fluorophore, FITC (fluorescein isothiocyanate), whose luminescence is insensitive to Hg(2+) was precoated on the fiber. Exposure to mercury quenched the red emission completely, and the green emission of the fiber appeared which was observed under dark field fluorescence microscopy. For the sensing experiment at the limit of sensitivity, we have used individual nanofibers. Quenching due to Hg(2+) ions was fast and uniform. Adaptation of such sensors to pH paper-like test-strips would make affordable water quality sensors at ultralow concentrations a reality.

Humidity sensitivity of metal ferrite nanoparticles [MFe 2 O 4 , M (II) ¼ Co, Cu, Mg, Ni and Zn] prepared by solid-state reaction of inorganic precursors was studied. The process was convenient, environmentally-friendly, inexpensive and efficient. The spinel structure of the compounds prepared by this method was confirmed by XRD and FT-IR studies. The surface morphology was observed by scanning electron microscopy, and the surface area was analysed by the nitrogen adsorption/desorption study by the Brunauer-Emmett-Teller (BET) method. All the samples were subjected to dc electrical conductivity studies at room temperature. The resistance measurements as a function of relative humidity in the range of 5-98% and the sensitivity factor (S f ¼ R 5% / R 98% ) were calculated. Among all the metal ferrites, zinc ferrite (ZnFe 2 O 4 ) possessed the highest humidity sensitivity factor of 2895 � 85, whereas the other compounds possessed a very low sensitivity factor. The response and recovery times of ZnFe 2 O 4 were 330 and 80 s.

Atomically precise gold and silver clusters are a new class of sensitizers which can be used as substitutes for dyes in the classical dye‐sensitized solar cells (DSCs). Here noble metal clusters protected by proteins and thiols (Au30@BSA, Au25SBB18, and Ag44MBA30) have been used for photovoltaic studies. These metal clusters were used as sensitizers for the photoanodes fabricated using TiO2 nanotubes (NTs) and the commercial P25 TiO2 nanoparticles. The TiO2, clusters and the solar cells were characterized by spectroscopy, microscopy, current‐voltage (I‐V) and incident photon‐to‐current conversion efficiency (IPCE) measurements. A systematic I‐V study revealed a conversion efficiency of 0.35 % for the Au30@BSA sensitized solar cell made from TiO2 NTs which showed an IPCE maximum of 3 % at ∼ 400 nm.

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