Chemosensing in microorganisms to practical biosensors

Microorganisms like bacteria can sense concentration of chemo-attractants in its medium very accurately. They achieve this through interaction between the receptors on their cell surface and the chemo-attractant molecules (like sugar). But the physical processes like diffusion set some limits on the accuracy of detection which was discussed by Berg and Purcell in the late seventies. We have a re-look at their work in order to assess what insight it may offer towards making efficient, practical biosensors. We model the functioning of a typical biosensor as a reaction-diffusion process in a confined geometry. Using available data first we characterize the system by estimating the kinetic constants for the binding/unbinding reactions between the chemo-attractants and the receptors. Then we compute the binding flux for this system which Berg and Purcell had discussed. But unlike in microorganisms where the interval between successive measurements determines the efficiency of the nutrient searching process, it turns out that biosensors depend on long time properties like signal saturation time which we study in detail. We also develop a mean field description of the kinetics of the system.Comment: 6 pages, 7 figure

Excited state deprotonation reactions of aromatic amines: a diffusion-controlled process

The excited state proton transfer reactions of carbazole (CAZL), indole (IND) and diphenylamine (DPA) were studied using a picosecond time-resolved technique. The forward rate constant for deprotonation shows a slight temperature dependence. The activation energy for the process was determined by taking various bases in both aqueous and non-aqueous media. It is of the order of the diffusional barrier in the solution phase. Hence it is concluded that the process is diffusion controlled. The values of the diffusion-controlled rate constants are discussed

One-and two-photon fluorescence excitation spectra of multi-chromophoric molecules

The splittings in S<SUB>1</SUB> states of molecules containing weakly coupled &#8719;-chromophoric groups have been determined through one-and two-photon fluorescence excitation (OPF and TPF) spectroscopy. The <SUP>1</SUP>B<SUB>2u</SUB> state of benzene splits by about 140 cm<SUP>-1</SUP> in the dimer, dihydroanthracene (DHA) and by about 2500 cm<SUP>-1</SUP> in the strained trimer, triptycene (TR). While the exciton splitting in the trans-dimer of acenaphthylene (TDA), in which the two naphthalene moieties are linked by two ethylene bridges, is only 130 cm<SUP>-1</SUP>, no such near degeneracies are noticed in two-methylene-bridge-linked dihydrodibenzanthracene (DBA) or dihydropentacene (DHP)

Circular dichroism spectrum of gadolinium diglycolate single crystal

One-photon absorption (OPA) and circular dichroism (CD) spectra of gadolinium diglycolate single crystal at room temperature are examined and analysed. Observed dipole strengths, rotational strengths and dissymmetry factors for <SUP>6</SUP>P<SUB>J</SUB> and <SUP>6</SUP>D<SUB>J</SUB> groups are compared with the calculated values and a satisfactory spectra-structure correlation between the observed and the simulated spectrum is achieved

Denbow Heath (X166508 - 35)

Denbow Heath (X166508 - 35)https://digitalmaine.com/mgs_geologic_field_photos/3588/thumbnail.jp

Investigating the Impact of Layer Thickness on Light Propagation in Photonic Crystals

Photonic crystals are a unique class of materials that have the ability to manipulate the flow of light through their periodic structures. In this study, we investigate the impact of layer thickness on light propagation in photonic crystals. We focus on six different material pairs: MgF2-TiO2, SiO2-TiO2, GeO2-TiO2, MgO-TiO2, Al2O3-TiO2, and ZnO-TiO2. The thickness of the layers was varied from 40 microns to 60 microns and from 55 microns to 100 microns. The results of the study show that as the thickness of the layers increases, the wavelength difference (bandgap) also increases. The greater the bandgap, the better the blocking performance. The best material for this variation was found to be MgF2-SiO2 with a thickness of around 60 nm and 105nm respectively. These findings provide important insights into the potential use of these materials in optical applications requiring wavelength filtering and blocking. The results of this study can be used to optimize the design of photonic crystals for specific optical applications, such as in telecommunications and sensing

Excited-state proton transfer kinetics of carbazole

Rate constants for the excited-state proton transfer reaction of carbazole in aqueous alkaline solution have been determined using picosecond single photon counting. Fluorescence decay measurements show that the back reaction is slow compared to the fluorescence decay time and therefore equilibrium is not attained in the excited state. The validity of a pK value for the lowest excited state determined from steady-state fluorescence measurements assuming equilibrium is discussed. It is concluded that the thermodynamic pK&#8727; value for carbazole is 10.98

Synthesis, mixed valence aspects and non-linear optical properties of the triruthenium complexes [{(bpy)<SUB>2</SUB>Ru<SUP>II</SUP>}<SUB>3</SUB>(L)]<SUP>3+</SUP> and [{(phen)<SUB>2</SUB>Ru<SUP>II</SUP>}<SUB>3</SUB>(L)]<SUP>3+</SUP>(bpy = 2,2-bipyridine, phen = 1,10-phenanthroline and L<SUP>3-</SUP>= 1,3,5-triazine-2,4,6-trithiol)

The triruthenium complexes [{(bpy)2RuII}3L]3+ [1]3+ and [{(phen)2RuII}3L]3+ [2]3+ have been synthesized via the reactions of [RuII(bpy)2(EtOH)2]2+ and [RuII(phen)2(EtOH)2]2+ with the trisodium salt of 1,3,5-triazine-2,4,6 trithiol (Na3L) respectively. In CH3CN, the complexes [1]3+ and [2]3+exhibit three reversible one-electron redox processes corresponding to successive Ru(II)/Ru(III) couples. The 190-250 mV separation in potential between the successive Ru(II)/Ru(III) couples is indicative of moderate intermetallic electronic coupling in the mixed valence states. The bipyridine and phenanthroline based reductions are observed at -1.58, -1.86 V and -1.77, -2.01, -2.43 V versus SCE respectively. The spectroelectrochemical study on the bipyridine derivative [1]n+ (n = 3-6) in acetonitrile medium at 243 κ shows a broad and relatively weak intervalence charge-transfer transition (IVCT) near 1900 nm for both the mixed valence states RuIIRuIIRuIII [1]4+ and RuIIRuIIIRuIII [1]5+, characteristic of class II behaviour. The calculated coupling constant (Vab), 560 cm-1 is also supportive of class II mixed-valence states. The electrochemically generated one-electron oxidised species [1]4+ or [2]4+ exhibits an EPR spectrum characteristic of low-spin RuIII ion in a distorted octahedral environment (g1 = 2.246, g2 = 1.993 for [1]4+ and g1 = 2.469, g2 = 2.191 for [2]4+). The complexes are moderately strongly luminescent at 77 κ . Both the complexes have also shown third order non-linear optical properties with γ = -4.5 × 10-29 esu for [1]3+ and -5.09 × 10-29 esu for [2]3+