Review of Dissolved CO and H2 Measurement Methods for Syngas Fermentation

Syngas fermentation is a promising technique to produce biofuels using syngas obtained through gasified biomass and other carbonaceous materials or collected from industrial CO-rich off-gases. The primary components of syngas, carbon monoxide (CO) and hydrogen (H2), are converted to alcohols and other chemicals through an anaerobic fermentation process by acetogenic bacteria. Dissolved CO and H2 concentrations in fermentation media are among the most important parameters for successful and stable operation. However, the difficulties in timely and precise dissolved CO and H2 measurements hinder the industrial-scale commercialization of this technique. The purpose of this article is to provide a comprehensive review of available dissolved CO and H2 measurement methods, focusing on their detection mechanisms, CO and H2 cross interference and operations in syngas fermentation process. This paper further discusses potential novel methods by providing a critical review of gas phase CO and H2 detection methods with regard to their capability to be modified for measuring dissolved CO and H2 in syngas fermentation conditions.

Graphite/gold nanoparticles electrode for direct protein attachment: characterization and gas sensing application

In this work, graphite/gold nanoparticles (G/AuNPs) were synthesized through a facile chemical method, and its potential application for direct protein attachment for electrochemical detection of carbon monoxide (CO) was investigated. The preparation of G/AuNPs electrodes was optimized by synthesizing the nanoparticles in different concentration of HAuCl₄.3H₂O at various temperatures. The G/AuNPs electrode was subsequently modified by four types of mercaptopropionic acid, including 1-mercaptopropionic, 3-mercaptopropionic, 6-mercaptopropionic, and 11-mercaptopropionic acid, to achieve the best structure for protein attachment. Visible absorption and electrochemical studies showed that 3-mercaptopropionic acid possesses the best performance regarding the electrical conductivity between electrode and protein redox center. The cyclic voltammetry results revealed that the modified electrode has an appropriate performance for CO detection at very low concentrations while keeping a linear response. The limit of detection for the modified electrode was calculated to be about 0.2 ppb. Finally, the interactions of cytochrome C and carbon monoxides were simulated using molecular dynamics (MD), and the effect of protein conformation changes on the electrochemical signal was thoroughly examined. The simulation results suggested that the proposed electrochemical sensor has an acceptable performance for the detection of CO due to less fluctuation of amino acids near the protein chain in the presence of CO molecules.

Imaging of compartmentalised intracellular nitric oxide, induced during bacterial phagocytosis, using a metalloprotein-gold nanoparticle conjugate

Nitric oxide (NO) plays an essential role within the immune system since it is involved in the break-down of infectious agents such as viruses and bacteria. The ability to measure the presence of NO in the intracellular environment would provide a greater understanding of the pathophysiological mechanism of this important molecule. Here we report the detection of NO from the intracellular phagolysosome using a fluorescently tagged metalloprotein-gold nanoparticle conjugate. The metalloprotein cytochrome c, fluorescently tagged with an Alexa Fluor dye, was self-assembled onto gold nanoparticles to produce a NO specific nanobiosensor. Upon binding of NO, the cytochrome c protein changes conformation which induces an increase of fluorescence intensity of the tagged protein proportional to the NO concentration. The nanobiosensor was sensitive to NO in a reversible and selective manner, and exhibited a linear response at NO concentrations between 1 and 300 μM. In RAW264.7γ NO- macrophage cells, the nanobiosensor was used to detect the presence of NO that had been endogenously generated upon stimulation of the cells with interferon-γ and lipopolysaccharide, or spontaneously released following treatment of the cells with a NO donor. Significantly, the nanobiosensor was shown to be taken up by the macrophages within phagolysosomes, i.e., the precise location where the NO, together with other species, destroys bacterial infection. The nanobiosensor measured, for the first time, increasing concentrations of NO produced during combined stimulation and phagocytosis of Escherichia coli bacteria from within localised intracellular phagolysosomes, a key part of the immune system.

Simplified procedure for encapsulating cytochrome c in silica aerogel nanoarchitectures while retaining gas-phase bioactivity

Cytochrome c (cyt. c) has been encapsulated in silica sol-gels and processed to form bioaerogels with gas-phase activity for nitric oxide through a simplified synthetic procedure. Previous reports demonstrated a need to adsorb cyt. c to metal nanoparticles prior to silica sol-gel encapsulation and processing to form aerogels. We report that cyt. c can be encapsulated in aerogels without added nanoparticles and retain structural stability and gas-phase activity for nitric oxide. While the UV-visible Soret absorbance and nitric oxide response indicate that cyt. c encapsulated with nanoparticles in aerogels remains slightly more stable and functional than cyt. c encapsulated alone, these properties are not very different in the two types of aerogels. From UV-visible and Soret circular dichroism results, we infer that cyt. c encapsulated alone self-organizes to reduce contact with the silica gel in a way that may bear at least some resemblance to the way cyt. c self-organizes into superstructures of protein within aerogels when nanoparticles are present. Both the buffer concentration and the cyt. c concentration of solutions used to synthesize the bioaerogels affect the structural integrity of the protein encapsulated alone within the dried aerogels. Optimized bioaerogels are formed when cyt. c is encapsulated from 40 mM phosphate buffered solutions, and when the loaded cyt. c concentration in the aerogel is in the range of 5 to 15 μM. Increased viability of cyt. c in aerogels is also observed when supercritical fluid used to produce aerogels is vented over relatively long times.

Immobilization of Myoglobin from Horse Skeletal Muscle in Hydrophilic Polymer Networks

This work examines the immobilization of myoglobin from horse skeletal muscle in hydrophilic polymer networks. Due to specific changes in the spectroscopic properties of hemoproteins during ligand binding, they could be employed in optical sensing devices. Two immobilization techniques were considered: imbibition and entrapment. Anionic hydrogels composed of methacrylic acid (MAA), cationic hydrogels composed of dimethylamino ethyl methacrylate (DMAEM), and neutral hydrogels composed of poly(ethylene glycol) monomethyl ether monomethacrylate (PEGMA; molecular weight = 200, 400, or 1000), all crosslinked with poly(ethylene glycol) dimethacrylate (PEGDMA) (molecular weight = 200, 600, or 1000), were synthesized by free-radical solution polymerization. By the imbibition method, MAA-based hydrogels incorporated the highest amount of myoglobin in comparison with PEGMA or DMAEM polymers. The evaluation of the correlation length of the networks revealed that MAA hydrogels had the highest correlation length in comparison with PEGMA-containing matrices or DMAEM hydrogels. Release experiments from MAA hydrogels at pHs 5.8 and 7.0 showed that the solute-transport mechanism was a combination of Fickian and chain relaxation diffusion. Myoglobin-loaded MAA hydrogels retained their heme reactivity after the immobilization process. The release of myoglobin incorporated by entrapment in MAA-PEGDMA hydrogels was highly influenced by the chain relaxation process. The diffusion coefficients of myoglobin incorporated by entrapment into anionic hydrogels were 2 orders of magnitude smaller (~10-13) than those for myoglobin incorporated by imbibition (10-11), both evaluated at pH 7.0. Substrate binding studies indicated that the protein biological activity was not compromised in those hydrogels loaded by the imbibition method, whereas prepolymeric solutions showed detrimental effects on protein stability.

Entrapment of fluorescent signaling DNA aptamers in sol-gel-derived silica

We report on the first successful immobilization of a DNA aptamer, in particular, a fluorescence-signaling DNA aptamer, within a sol-gel-derived matrix. The specific aptamer examined in this study undergoes a structural switch in the presence of adenosine triphosphate (ATP) to release a dabcyl-labeled nucleotide strand (QDNA), which in turn relieves the quenching of a fluorescein label that is also present in the aptamer structure. It was demonstrated that aptamers containing a complementary QDNA strand along with either a short complimentary strand bearing fluorescein (tripartite structure) or a directly bound fluorescein moiety (bipartite structure) remained intact upon entrapment within biocompatible sol-gel derived materials and retained binding activity, structure-switching capabilities, and fluorescence signal generation that was selective and sensitive to ATP concentration. Studies were undertaken to evaluate the properties of the immobilized aptamers that were either in their native state or bound to streptavidin using a terminal biotin group on the aptamer, including response time, accessibility, and leaching. Furthermore, signaling abilities were optimized through evaluation of different QDNA constructs. These studies indicated that the aptamers remained in a state that was similar to solution, with moderate leaching, only minor decreases in accessibility to ATP, and an expected reduction in response time due to diffusional barriers to mass transport of the analyte through the silica matrix. Entrapment of the aptamer also resulted in protection of the DNA against degradation from nucleases, improving the potential for use of the aptamer for in vivo sensing. This work demonstrates that sol-gel-derived materials can be used to successfully immobilize and protect DNA-based biorecognition elements and, in particular, DNA aptamers, opening new possibilities for the development of DNA aptamer-based devices, such as affinity columns, microarrays, and fiber-optic sensors.

Fluorescence-Based Siderophore Biosensor for the Determination of Bioavailable Iron in Oceanic Waters

With direct evidence that iron is the chemical limitation of phytoplankton growth, particularly in the Southern Ocean, it is increasingly important to develop new tools that provide direct measurement of the bioavailable iron fraction in oceanic waters. Here we report the development of a fluorescence quenching-based siderophore biosensor capable of the in situ measurement of this ultratrace Fe(III) fraction at ambient pH ( approximately 8). Parabactin was extracted from cultures of Paracoccus denitrificans. The purified siderophore was encapsulated within a spin-coated sol-gel thin film, which was subsequently incorporated in a flow cell system. The parabactin biosensor has been fully characterized for the detection of Fe(III) in seawater samples. The biosensor can be regenerated by lowering the pH of the flowing solution, thereby releasing the chelated Fe(III), enabling multiple use. The LOD of the biosensor was determined to be 40 pM, while for an Fe(III) concentration of 1 nM, a reproducibility with a RSD of 6% (n = 10) was obtained. The accuracy of the biosensing system has been determined through analysis of a certified seawater reference sample. Samples from the Atlantic Ocean have been analyzed using the parabactin biosensor providing a concentration vs depth profile for the bioavailable Fe(III) fraction in the 50 pM-1 nM range.

Nanovolume Kinase Inhibition Assay Using a Sol−Gel-Derived Multicomponent Microarray

We report on the development of a new class of kinase microarrays based on the coimmobilization of both kinase and substrate components within a single pin-printed sol-gel microarray element and the use of such arrays for nanovolume inhibition assays. We successfully immobilized the alpha-catalytic subunit of cAMP-dependent protein kinase (PKA) and the peptide substrate kemptide within sol-gel-derived microarrays for the purpose of monitoring phosphorylation and inhibition. Using Pro-Q Diamond stain as an end-point indicator of phosphorylation, we demonstrate the selective detection of phosphoproteins over nonphosphorylated controls and the ability to detect phosphorylated proteins over a 500-fold concentration range. Limits of detection for the phosphoprotein beta-casein were 7.5 pg, and the detectable signal remained linear up to 3.75 ng of protein per array spot. PKA is demonstrated to be active when coentrapped with two different substrates, and inhibition assays for PKA with the inhibitors H7 and H89 demonstrate the ability to detect kinase inhibition as well as derive IC50 plots from a single array using an overprinting method to deliver approximately 0.6 nL of reagent per array element, or a total of 72 nL of reagents to generate a full, 12-point IC50 curve in pentuplicate.

Immobilization of active hydrogenases by encapsulation in polymeric porous gels

Hydrogenases encapsulated in porous polymeric silica gels retain significant levels of hydrogen production activity when compared to hydrogenases in solution using reduced methyl viologen as an electron donor. Encapsulated hydrogenases remain active after storage at room temperature for longer than four weeks and are less sensitive to proteolytic digestion. Nanoscopic confinement of active hydrogenases in solids paves the way for their potential use in hydrogen producing catalytic materials applications.

Entrapment of Src Protein Tyrosine Kinase in Sugar-Modified Silica

A novel sugar-modified silica has been used to entrap for the first time a protein tyrosine kinase (PTK). Silane precursors bearing covalently attached gluconamide moieties were used in combination with the biocompatible precursor diglycerylsilane (DGS) to generate sol-gel derived silica that was able to encapsulate highly active Src PTK and preserve the activity of the enzyme over multiple uses. The relative activity of the enzyme was assayed using a LANCE based fluorescence resonance energy transfer method involving time-gated detection of fluorescence from a europium labeled antiphosphotyrosine antibody and Cy5 labeled streptavidin upon mutual binding to biotinylated phosphopeptides. Using this detection method, with the antibody and streptavidin external to the sol-gel matrix, it was possible to detect the phosphorylation of peptides with molecular weights of up to 2300 Da using the entrapped enzyme in N-(3-triethoxysilylpropyl)gluconamide (GLTES) doped glasses. Src kinase-doped glasses, derived from precursors such as tetramethyl orthosilicate, tetraethyl orthosilicate, or DGS that did not contain GLTES, provided no detectable enzyme activity. The addition of 1 mM ATP to the GLTES/DGS sol before the encapsulation of the protein increased the activity of the enzyme in the resulting gel, likely through a ligand-based stabilization mechanism. The use of such a system for determination of PTK activity and inhibition is demonstrated, setting the stage for the development of chromatographic and microarray based methods for the screening of kinase inhibitors.

Protein-Doped Monolithic Silica Columns for Capillary Liquid Chromatography Prepared by the Sol−Gel Method: Applications to Frontal Affinity Chromatography

The development of bioaffinity chromatography columns that are based on the entrapment of biomolecules within the pores of sol-gel-derived monolithic silica is reported. Monolithic nanoflow columns are formed by mixing the protein-compatible silica precursor diglycerylsilane with a buffered aqueous solution containing poly(ethylene oxide) (PEO, MW 10,000) and the protein of interest and then loading this mixture into a fused-silica capillary (150-250-microm i.d.). Spinodal decomposition of the PEO-doped sol into two distinct phases prior to the gelation of the silica results in a bimodal pore distribution that produces large macropores (>0.1 microm), to allow good flow of eluent with minimal back pressure, and mesopores (approximately 3-5-nm diameter) that retain a significant fraction of the entrapped protein. Addition of low levels of (3-aminopropyl)triethoxysilane is shown to minimize nonselective interactions of analytes with the column material, resulting in a column that is able to retain small molecules by virtue of their interaction with the entrapped biomolecules. Such columns are shown to be suitable for pressure-driven liquid chromatography and can be operated at relatively high flow rates (up to 500 microL x min(-1)) or with low back pressures (<100 psi) when used at flow rates of 5-10 microL x min(-1). The clinically relevant enzyme dihydrofolate reductase was entrapped within the bioaffinity columns and was used to screen mixtures of small molecules using frontal affinity chromatography with mass spectrometric detection. Inhibitors present in compound mixtures were retained via bioaffinity interactions, with the retention time being dependent on both the ligand concentration and the affinity of the ligand for the protein. The results suggest that such columns may find use in high-throughput screening of compound mixtures.

Electrochemical control of protein monolayers at indium tin oxide surfaces for the reagentless optical biosensing of nitric oxide

Cytochrome c has been immobilized onto functionalized, optically transparent indium tin oxide (ITO) electrodes by covalent and electrostatic techniques. Covalent immobilization was achieved by the formation of a disulfide bond between N-succinimidyl 3-(2-pyridyldithio)propionate-(SPDP-) modified cytochrome c and SPDP-silanized ITO. Additionally, ITO electrodes have been modified with the bifunctional reagent 1,12-dodecanedicarboxylic acid (DDCA), resulting in formation of a carboxylic acid-terminated monolayer. Covalent protein attachment to the DDCA-functionalized ITO was achieved with the cross-linker 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride. Electrostatic attachment of the protein involved ion-pair and hydrogen-bond interactions between the terminating carboxylic acid groups of the DDCA-functionalized ITO and the primary amine groups of the lysine residues of cytochrome c. The electrostatic interaction between the cytochrome c and the functionalized ITO resulted in greater rotational mobility of the protein at the electrode surface, leading to ca. 63% electroactivity, as compared to ca. 41% electroactivity for the covalently immobilized protein. The redox state of the electrostatically bound cytochrome c monolayers could be electrochemically switched between ferric and ferrous forms. Electrochemical control of the bound protein was used to regenerate the biosensing surface following binding of nitric oxide (NO). Ligation of NO with the cytochrome c was monitored by measurement of the change of absorbance intensity at 416 nm. Through application of a negative potential, the cytochrome c was reduced from the ferric to the ferrous form, which led to the removal of the ligated NO. Application of a positive potential regenerated the ferric cytochrome c, enabling multiple repeat measurements of NO. Such electrochemical control of proteins immobilized on transparent electrodes enables the optical biosensing of analyte targets without recourse to exogenous reagents.

Screening of inhibitors using enzymes entrapped in sol-gel-derived materials

In recent years, a number of new methods have been reported that make use of immobilized enzymes either on microarrays or in bioaffinity columns for high-throughput screening of compound libraries. A key question that arises in such methods is whether immobilization may alter the intrinsic catalytic and inhibition constants of the enzyme. Herein, we examine how immobilization within sol-gel-derived materials affects the catalytic constant (kcat), Michaelis constant (KM), and inhibition constant (KI) of the clinically relevant enzymes Factor Xa, dihydrofolate reductase, cyclooxygenase-2, and gamma-glutamyl transpeptidase. These enzymes were encapsulated into sol-gel-derived glasses produced from either tetraethyl orthosilicate (TEOS) or the newly developed silica precursor diglyceryl silane (DGS). It was found that the catalytic efficiency and long-term stability of all enzymes were improved upon entrapment into DGS-derived materials relative to entrapment in TEOS-based glasses, likely owing to the liberation of the biocompatible reagent glycerol from DGS. The KM values of enzymes entrapped in DGS-derived materials were typically higher than those in solution, whereas upon entrapment, kcat values were generally lowered by a factor of 1.5-7 relative to the value in solution, indicating that substrate turnover was limited by partitioning effects or diffusion through the silica matrix. Nonetheless, the apparent KI value for the entrapped enzyme was in most cases within error of the value in solution, and even in the worst case, the values differed by no more than a factor of 3. The implications of these findings for high-throughput screening are discussed.

Preparation of porous titania sol-gel matrix for immobilization of horseradish peroxidase by a vapor deposition method

A new and facile vapor deposition method has been developed for the preparation of sol-gel matrix. This method was used to form a titania sol-gel thin film and to immobilize horseradish peroxidase (HRP) on a glassy carbon electrode surface for the production of an amperometric hydrogen peroxide biosensor. This process prevented the cracking of conventional sol-gel-derived glasses. The morphologies of both titania sol-gel and the enzyme membranes were characterized using scanning electron microscopy and proved to be chemically clean, porous, and homogeneous and to have a very narrow particle size distribution. The sol-gel-derived titania-modified electrode retained the enzyme bioactivity and provided for long-term stability of the enzyme in storage. In the presence of catechol as a mediator, the sensor exhibited a rapid electrocatalytic response (less than 5 s), a linear calibration range from 0.08 to 0.56 mM with a detection limit of 1.5 microM and a high sensitivity (61.5 microA mM(-1)) for monitoring of H2O2. Effects of pH and operating potential were also explored for optimum analytical performance by using the amperometric method. The apparent Michaelis-Menten constant of the encapsulated HRP was 1.89 +/- 0.21 mM.

Sol-gel encapsulated horseradish peroxidase: a catalytic material for peroxidation

This study addresses the viability of sol-gel encapsulated HRP (HRP:sol-gel) as a recyclable solid-state catalytic material. Ferric, ferric-CN, ferrous, and ferrous-CO forms of HRP:sol-gel were investigated by resonance Raman and UV-visible methods. Electronic and vibrational spectroscopic changes associated with changes in spin state, oxidation state, and ligation of the heme in HRP:sol-gel were shown to correlate with those of HRP in solution, showing that the heme remains a viable ligand-binding complex. Furthermore, the high-valent HRP:sol-gel intermediates, compound I and compound II, were generated and identified by time-resolved UV-visible spectroscopy. Catalytic activity of the HRP:sol-gel material was demonstrated by enzymatic assays by using I(-), guaiacol, and ABTS as substrates. Encapsulated HRP was shown to be homogeneously distributed throughout the sol-gel host. Differences in turnover rates between guaiacol and I(-) implicate mass transport of substrate through the silicate matrix as a defining parameter in the peroxidase activity of HRP:sol-gel. HRP:sol-gel was reused as a peroxidation catalyst for multiple reaction cycles without loss of activity, indicating that such materials show promise as reusable catalytic materials.

Purification and magneto-optical spectroscopic characterization of cytoplasmic membrane and outer membrane multiheme c-type cytochromes from Shewanella frigidimarina NCIMB400

Two membranous c-type cytochromes from the Fe(III)-respiring bacterium Shewanella frigidimarina NCIMB400, CymA and OmcA, have been purified and characterized by UV-visible, magnetic circular dichroism, and electron paramagnetic resonance spectroscopies. The 20-kDa CymA is a member of the NapC/NirT family of multiheme cytochromes, which are invariably anchored to the cytoplasmic membrane of Gram-negative bacteria, and are postulated to mediate electron flow between quinols and periplasmic redox proteins. CymA was found to contain four low-spin c-hemes, each with bis-His axial ligation, and midpoint reduction potentials of +10, -108, -136, and -229 mV. The 85-kDa OmcA is located at the outer membrane of S. frigidimarina NCIMB400, and as such might function as a terminal reductase via interaction with insoluble Fe(III) substrates. This putative role is supported by the finding that the protein was released into solution upon incubation of harvested intact cells at 25 degrees C, suggesting an attachment to the exterior face of the outer membrane. OmcA was revealed by magneto-optical spectrocopies to contain 10 low-spin bis-His ligated c-hemes, with the redox titer indicating two sets of near iso-potential components centered at -243 and -324 mV.

Cellular applications of a sensitive and selective fiber-optic nitric oxide biosensor based on a dye-labeled heme domain of soluble guanylate cyclase

Nitric oxide-selective sensors have been prepared with the heme domain of soluble guanylate cyclase (sGC), the only known receptor for signal transduction involving nitric oxide. Expressed in and purified from E. coli, the heme domain contains a stoichiometric amount of heme that has electronic and resonance Raman spectra almost identical to those of heterodimeric (native) sGC purified from bovine lung. The small size of the heme domain, its inability to bind oxygen, and its high affinity for nitric oxide make it well-suited for sensor applications. The heme domain has been labeled with a fluorescent reporter dye and changes in this dye's intensity are observed based on the sGC heme domain's characteristic binding of nitric oxide. The current sensors are prepared with 100-microns optical fiber but could also be prepared using submicrometer fiber tips. These sensors have fast, linear, and reversible responses to nitric oxide and are unaffected by numerous common interferents, such as oxygen, nitrite and nitrate. The sensor limit of detection is 1 microM nitric oxide. Glutathione has been shown to decrease the sensitivity of the sensor; however, the sensor response remains linear and can be calibrated on the basis of the glutathione concentration present in the biological environment of interest. The sensors have been used to measure extracellular nitric oxide production by BALB/c mouse macrophages. Minimal nitric oxide was produced by untreated cells, while high levels of nitric oxide were released from activated cells, e.g., 111 +/- 2 microM in a given cell culture.

Ratiometric and fluorescence-lifetime-based biosensors incorporating cytochrome c' and the detection of extra- and intracellular macrophage nitric oxide

Ratiometric and lifetime-based sensors have been designed for cellular detection of nitric oxide. These sensors incorporate cytochrome c', a hemoprotein known to bind nitric oxide selectively. The cytochrome c' is labeled with a fluorescent reporter dye, and changes in this dye's intensity or fluorescence lifetime are observed as the protein binds nitric oxide. The ratiometric sensors are composed of dye-labeled cytochrome c' attached to the optical fiber via colloidal gold, along with fluorescent microspheres as intensity standards. These ratiometric sensors exhibit linear response, have fast response times (< or = 0.25 s), and are completely reversible. The sensors are selective over numerous common interferents such as nitrite, nitrate, and oxygen species, and the limit of detection is 8 microM nitric oxide. The lifetime-based measurements are made using free, dye-labeled cytochrome c' in solution and have a limit of detection of 30 microM nitric oxide. The use of these two techniques has allowed measurement of intra- and extracellular macrophage nitric oxide. Employing the ratiometric fiber sensors gave a multicell culture average extracellular nitric oxide concentration of 210 +/- 90 microM for activated macrophages, while an average intracellular concentration of 160 +/- 10 microM was determined from the lifetime-based measurements of dye-labeled cytochrome c' in the macrophage cytosol. Microscopic adaptation of the lifetime-based methods described here would allow direct correlation of intracellular nitric oxide levels with specific cellular activities, such as phagocytosis.

Chemical and biochemical sensors based on advances in materials chemistry

The advance of materials chemistry has influenced the design of analytical sensors, especially those using spectroscopic or electrochemical methods for generating the signal. New methods of immobilizing enzymes, chromophores, and electron-transfer catalysts have resulted from initiatives in materials science. Systems based on sol-gel chemistry are especially noteworthy in this regard, but other important materials for chemical and biochemical sensors include zeolites, organic polymers, and various conducting composites. Applications cited include determinations of inorganic ions, gases, neurotransmitters, alcohols, carbohydrates, amino acids, proteins, and DNA.

Development and cellular applications of fiber optic nitric oxide sensors based on a gold-adsorbed fluorophore

A new design for optochemical sensors has been applied to the development of a nitric oxide selective fiber optic sensor. This sensor is composed of a fluorescein derivative dye attached to colloidal gold. The fluorescein dye rearranges as nitric oxide adsorbs onto the gold, inducing a decrease in the fluorescence intensity of the dye. This mechanism has allowed preparation of fiber optic dye-based nitric oxide sensors, which have been made ratio-metric by addition of reference dye microspheres. Previously developed fast, selective optical sensors for detection of aqueous nitric oxide involved a protein, such as cytochrome c'. The new fluorescein derivative chemical sensors have characteristics similar to those of the protein-based biosensors, including fast response times, excellent selectivity, and complete reversibility. In addition, the chemical sensors have advantages such as greater stability and commercially available components. These sensors were utilized to measure nitric oxide production by BALB/c mouse macrophages.

Fiber-optic nitric oxide-selective biosensors and nanosensors

Fiber-optic biosensors that are selective for nitric oxide and do not respond to most potential interferents have been prepared with cytochromes c'. Both micro- and nanosensors have been prepared, and their response is fast (< 1 s), reversible, and linear up to 1 mM nitric oxide. The detection limit is 20 microM, making the sensor useful for some biological samples, such as the macrophages studied here. While sensors have been prepared based on the fluorescence of the cytochromes c', optodes with greatly enhanced signal-to-noise ratios have been made by labeling the cytochrome c' with a fluorescent dye. Comparisons of cytochromes c' from three species of bacteria as well as of two matrixes were performed and the optimum sensor configuration is described.

T state hemoglobin binds oxygen noncooperatively with allosteric effects of protons, inositol hexaphosphate, and chloride

Hemoglobin is the paradigm of allosteric proteins. Over the years, cooperative oxygen binding has been explained by different models predicting that the T state of hemoglobin binds oxygen either noncooperatively or with some degree of cooperativity or with strong cooperativity. Therefore, a critical test that discriminates among models is to determine the oxygen binding by the T state of hemoglobin. Fixation of hemoglobin in the T state has been achieved either by crystallization from polyethylene glycol solutions or by encapsulation in wet porous silica gels. Hemoglobin crystals bind oxygen noncooperatively with reduced affinity compared with solution, with no Bohr effect and with no influence of other allosteric effectors. In this study, we have determined accurate oxygen-binding curves to the T state of hemoglobin in silica gels with the same microspectrophotometric apparatus and multiwavelengths analysis used in crystal experiments. The T state of hemoglobin in silica gels binds oxygen noncooperatively with an affinity and a Bohr effect similar to those observed in solution for the binding of the first oxygen molecule. Other allosteric effectors such as inositol hexaphosphate, bezafibrate, and chloride significantly affect oxygen affinity. Therefore, T state hemoglobins that are characterized by strikingly different functional properties share the absence of cooperativity in the binding of oxygen. These findings are fully consistent with the Monod, Wyman, and Changeux model and with most features of Perutz's stereochemical model, but they are not consistent with models of both Koshland and Ackers.

Calcium biosensing with a sol-gel immobilized photoprotein

Aequorin, the bioluminescent protein found in the jellyfish Aequorea sp., has been immobilized in a porous sol-gel glass environment. The luminescence from this protein is specifically triggered by the presence of calcium ions, thus offering exciting possibilities for the development of an optical biosensor for this cationic species. The luminescence emission spectrum has been measured from the aequorin protein after interaction with calcium ions. The intensity of the luminescence, measured at the peak maximum of 470 nm, for the encapsulated protein has been calibrated against calcium ion concentration. The characterization of the protein within the sol-gel matrix has been reported together with biosensing experiments using human sera and milk samples. The results suggest that the sol-gel encapsulated aequorin protein offers potential as a one shot bioluminescence based biosensor for the determination of calcium ions in such complex matrices.