Heme Oxygenase-1 Expression in Dendritic Cells Contributes to Protective Immunity against Herpes Simplex Virus Skin Infection.

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) infections are highly prevalent in the human population and produce mild to life-threatening diseases. These viruses interfere with the function and viability of dendritic cells (DCs), which are professional antigen-presenting cells that initiate and regulate the host's antiviral immune responses. Heme oxygenase-1 (HO-1) is an inducible host enzyme with reported antiviral activity against HSVs in epithelial cells and neurons. Here, we sought to assess whether HO-1 modulates the function and viability of DCs upon infection with HSV-1 or HSV-2. We found that the stimulation of HO-1 expression in HSV-inoculated DCs significantly recovered the viability of these cells and hampered viral egress. Furthermore, HSV-infected DCs stimulated to express HO-1 promoted the expression of anti-inflammatory molecules, such as PDL-1 and IL-10, and the activation of virus-specific CD4+ T cells with regulatory (Treg), Th17 and Treg/Th17 phenotypes. Moreover, HSV-infected DCs stimulated to express HO-1 and then transferred into mice, promoted the activation of virus-specific T cells and improved the outcome of HSV-1 skin infection. These findings suggest that stimulation of HO-1 expression in DCs limits the deleterious effects of HSVs over these cells and induces a favorable virus-specific immune response in the skin against HSV-1.

Functionalized graphene quantum dots obtained from graphene foams used for highly selective detection of Hg2+ in real samples.

Here we report the use of graphene quantum dots (GQDs), obtained from 3D graphene foam, functionalized with 8-hydroxyquinoline (8-HQ) for the sensitive and selective detection of Hg2+ via front-face fluorescence. The great surface area and active groups within the GQDs permitted the functionalization with 8-HQ to increase their selectivity toward the analyte of interest. The fluorescence probe follows the Stern-Volmer model, yielding a direct relationship between the degree of quenching and the concentration of the analyte. Diverse parameters, including the pH and the use of masking agents, were optimized in order to improve the selectivity toward Hg2+ down to a limit of detection of 2.4 nmol L-1. It is hereby demonstrated that the functionalized GQDs work perfectly fine under adverse conditions such as acidic pH and in the presence of a large number of cationic and anionic interferences for the detection of Hg2+ in real samples. Parallel measurements using cold vapor atomic fluorescence spectrometry also demonstrated an excellent correlation with the front-face fluorescence method applied here for real samples including tap, river, underground, and dam waters.

Pharmacological Inhibition of IRE-1 Alpha Activity in Herpes Simplex Virus Type 1 and Type 2-Infected Dendritic Cells Enhances T Cell Activation.

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) infections are life-long and highly prevalent in the human population. These viruses persist in the host, eliciting either symptomatic or asymptomatic infections that may occur sporadically or in a recurrent manner through viral reactivations. Clinical manifestations due to symptomatic infection may be mild such as orofacial lesions, but may also translate into more severe diseases, such as ocular infections that may lead to blindness and life-threatening encephalitis. A key feature of herpes simplex viruses (HSVs) is that they have evolved molecular determinants that hamper numerous components of the host's antiviral innate and adaptive immune system. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), by inhibiting their T cell-activating capacity and eliciting their apoptosis after infection. Previously, we reported that HSV-2 activates the splicing of the mRNA of XBP1, which is related to the activity of the unfolded protein response (UPR) factor Inositol-Requiring Enzyme 1 alpha (IRE-1α). Here, we sought to evaluate if the activation of the IRE-1α pathway in DCs upon HSV infection may be related to impaired DC function after infection with HSV-1 or HSV-2. Interestingly, the pharmacological inhibition of the endonuclease activity of IRE-1α in HSV-1- and HSV-2-infected DCs significantly reduced apoptosis in these cells and enhanced their capacity to migrate to lymph nodes and activate virus-specific CD4+ and CD8+ T cells. These findings suggest that the activation of the IRE-1α-dependent UPR pathway in HSV-infected DCs may play a significant role in the negative effects that these viruses exert over these cells and that the modulation of this signaling pathway may be relevant for enhancing the function of DCs upon infection with HSVs.

Simultaneous Detection and Photocatalysis Performed on a 3D Graphene/ZnO Hybrid Platform.

The synergy between graphene foam (Gf) and ZnO nanoparticles (NPs) allows the detection of analytes at low conentrations, which can be subsequently photocatalyzed on the hybrid surface as well as in the liquid phase upon illumination with low-power UV-vis light-emitting diode (LED) lamps. Detection of methylene blue (MB) and bisphenol A (BPA) is monitored either by graphene-enhanced Raman scattering (GERS) or molecular doping/sensing upon analyte adsorption. Using GERS, we were able to detect concentrations as low as 0.3 ppm of MB, which remained adsorbed on the graphene surface after a photocatalytic conversion of 88% (total conversion). The photocatalysis performances of BPA and MB performed in the liquid phase were lower and corresponded to 73 and 33% as indicated by gas chromatography-mass spectrometry (GC/MS) and UV-vis, respectively. The kinetics of photocatalysis was fitted with a quasi-first-order reaction, and the apparent rate constant (kapp) was calculated according to the Langmuir-Hinshelwood model. The fastest kinetics was achieved with the hybrid platform named "Gf-ZnO400", which was thermally treated at high temperatures and with most of the Ni etched away. This is consistent with the excellent electronic interaction between ZnO and graphene foam as indicated by photoelectrochemistry analysis. We mainly employed Raman scattering and UV-vis spectroscopy analyses for detection and photocatalysis applications; however, we also used other complementary techniques such as focused ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance, GC/MS, and photoelectrochemistry to explore the synergetic behavior of these two nanomaterials. This work brings about new insights into the detection of analyte molecules followed by photocatalysis performed in the solid and liquid states.

Hybrid ionic liquid-3D graphene-Ni foam for on-line preconcentration and separation of Hg species in water with atomic fluorescence spectrometry detection.

A preconcentration method based on a novel hybrid sorption nanomaterial consisting in a 3D graphene-Ni foam functionalized with an ionic liquid (IL) was developed for Hg species determination. The capability of different phosphonium-ionic liquids (PILs) to functionalize the hybrid material and form ion-pairs with Hg(II) chlorocomplex was evaluated. A comparative study with PILs containing the trihexyl(tetradecyl)phosphonium cation but different anions (dicyanamide and decanoate) and trihexyl(tetradecyl)phosphonium chloride was performed. Inorganic Hg (InHg) species was selectively retained forming a chlorocomplex (HgCl42-) followed by its retention in a column filled with the IL-3D graphene-Ni foam material implemented in an on-line micro solid phase extraction system (µSPE). The elution of Hg from the column was performed with SnCl2, which was also used as reducing agent for subsequent detection by cold vapor generation atomic fluorescence spectrometry (CV-AFS). Organic mercury species (OrgHg) were not retained on the sorption material of the column. The effect of several parameters determining the efficiency of the preconcentration and detection system (sample loading and elution flow rate, sample volume, instrumental conditions, etc.) was investigated. A sensitivity enhancement factor (EF) of 180 was achieved for InHg. The detection limit obtained after the preconcentration of 100 mL of sample was 3.6 ng L-1 of InHg. The relative standard deviation (RSD) was 4.1% (at 1 µg L-1 InHg and n = 10) calculated from the peak height of the absorbance signals (Gaussian form and reproducible peaks). This work reports the first application of the IL-3D graphene-Ni foam in an on-line µSPE preconcentration system for the speciation analysis of Hg in mineral, tap, and river water samples.

[Handheld laser maculopathy: two case reports]. / Retinopatía por puntero láser: presentación de dos casos.

The improper use of laser pointers, especially for recreational use in children, may cause sight-threatening retinal injuries. The retinal damage it is not well characterized because most publications are isolated cases or small series. The treatment and visual prognosis are variable according to the morphology of the macular damage. In some cases, there is no treatment, and spontaneous healing can be developed; however, others require surgery. In a series of 13 cases, 1 required surgery and the rest observation; 3 patients obtained a spontaneous healing with visual acuity of 10/10. We describe two patients aged 16 and 12 years with decreased visual acuity. Macular alterations appear in the fundus of the eye. The visual acuity of both recovered completely without treatment.

El uso indebido de los punteros láser, especialmente el recreativo en los niños, puede causar lesiones retinianas que amenazan la visión. El daño retiniano que producen no está bien caracterizado, debido a que la mayoría de las publicaciones son casos aislados o series pequeñas. El tratamiento y el pronóstico visual es variable según la morfología del daño macular que presenten. En algunos casos, no existe tratamiento, y pueden evolucionar a la curación espontánea; sin embargo, otros precisan cirugía. En una serie de 13 casos, uno requirió cirugía, mientras que el resto necesitó solo observación. De estos 13 casos, 3 pacientes se curaron en forma espontánea, con agudeza visual de 10/10. Se describen dos pacientes de 16 y 12 años con disminución de agudeza visual tras el uso indebido de punteros láser. En el fondo de ojo, presentaron alteraciones maculares. La agudeza visual de ambos se recuperó completamente sin tratamiento.

Experimental Dissection of the Lytic Replication Cycles of Herpes Simplex Viruses in vitro.

Herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) produce lifelong infections and are highly prevalent in the human population. Both viruses elicit numerous clinical manifestations and produce mild-to-severe diseases that affect the skin, eyes, and brain, among others. Despite the existence of numerous antivirals against HSV, such as acyclovir and acyclovir-related analogs, virus variants that are resistant to these compounds can be isolated from immunosuppressed individuals. For such isolates, second-line drugs can be used, yet they frequently produce adverse side effects. Furthermore, topical antivirals for treating cutaneous HSV infections usually display poor to moderate efficacy. Hence, better or novel anti-HSV antivirals are needed and details on their mechanisms of action would be insightful for improving their efficacy and identifying specific molecular targets. Here, we review and dissect the lytic replication cycles of herpes simplex viruses, discussing key steps involved in cell infection and the processes that yield new virions. Additionally, we review and discuss rapid, easy-to-perform and simple experimental approaches for studying key steps involved in HSV replication to facilitate the identification of the mechanisms of action of anti-HSV compounds.

Immunization with a Mixture of Nucleoprotein from Human Metapneumovirus and AbISCO-100 Adjuvant Reduces Viral Infection in Mice Model.

The human metapneumovirus (hMPV) is the second leading cause globally of acute infection of the respiratory tract in children, infecting the upper and lower airways. The hMPV may induce an inappropriate Th2-type immune response, which causes severe pulmonary inflammation, leading to the obstruction of airways. Despite its severe epidemiological relevance, no vaccines are currently available for the prevention of hMPV-induced illness. In this investigation, we demonstrated that immunization of mice with the recombinant hMPV nucleoprotein (hMPV-N) mixed with the AbISCO-100 adjuvant reduced viral replication in lungs following challenge with the virus. We found that immunized mice had reduced weight loss, decreased granulocytes in the lung, an increased level of specific nucleoprotein antibodies of IgG1 and IgG2a-isotypes, and a local profile of Th1/Th17-type cytokines. Our results suggest that immunization with the hMPV-N and the AbISCO-100 adjuvant induces a reduction of viral infection and could be considered for the development of an hMPV vaccine.

Pharmacological Induction of Heme Oxygenase-1 Impairs Nuclear Accumulation of Herpes Simplex Virus Capsids upon Infection.

Heme oxygenase-1 (HO-1) is an inducible enzyme that is expressed in response to physical and chemical stresses, such as ultraviolet radiation, hyperthermia, hypoxia, reactive oxygen species (ROS), as well as cytokines, among others. Its activity can be positively modulated by cobalt protoporphyrin (CoPP) and negatively by tin protoporphirin (SnPP). Once induced, HO-1 degrades iron-containing heme into ferrous iron (Fe2+), carbon monoxide (CO) and biliverdin. Importantly, numerous products of HO-1 are cytoprotective with anti-apoptotic, anti-oxidant, anti-inflammatory, and anti-cancer effects. The products of HO-1 also display antiviral properties against several viruses, such as the human immunodeficiency virus (HIV), influenza, hepatitis B, hepatitis C, and Ebola virus. Here, we sought to assess the effect of modulating HO-1 activity over herpes simplex virus type 2 (HSV-2) infection in epithelial cells and neurons. There are no vaccines against HSV-2 and treatment options are scarce in the immunosuppressed, in which drug-resistant variants emerge. By using HSV strains that encode structural and non-structural forms of the green fluorescent protein (GFP), we found that pharmacological induction of HO-1 activity with CoPP significantly decreases virus plaque formation and the expression of virus-encoded genes in epithelial cells as determined by flow cytometry and western blot assays. CoPP treatment did not affect virus binding to the cell surface or entry into the cytoplasm, but rather downstream events in the virus infection cycle. Furthermore, we observed that treating cells with a CO-releasing molecule (CORM-2) recapitulated some of the anti-HSV effects elicited by CoPP. Taken together, these findings indicate that HO-1 activity interferes with the replication cycle of HSV and that its antiviral effects can be recapitulated by CO.

Evasion of early antiviral responses by herpes simplex viruses.

Besides overcoming physical constraints, such as extreme temperatures, reduced humidity, elevated pressure, and natural predators, human pathogens further need to overcome an arsenal of antimicrobial components evolved by the host to limit infection, replication and optimally, reinfection. Herpes simplex virus-1 (HSV-1) and herpes simplex virus-2 (HSV-2) infect humans at a high frequency and persist within the host for life by establishing latency in neurons. To gain access to these cells, herpes simplex viruses (HSVs) must replicate and block immediate host antiviral responses elicited by epithelial cells and innate immune components early after infection. During these processes, infected and noninfected neighboring cells, as well as tissue-resident and patrolling immune cells, will sense viral components and cell-associated danger signals and secrete soluble mediators. While type-I interferons aim at limiting virus spread, cytokines and chemokines will modulate resident and incoming immune cells. In this paper, we discuss recent findings relative to the early steps taking place during HSV infection and replication. Further, we discuss how HSVs evade detection by host cells and the molecular mechanisms evolved by these viruses to circumvent early antiviral mechanisms, ultimately leading to neuron infection and the establishment of latency.

Synergy between graphene and Au nanoparticles (heterojunction) towards quenching, improving Raman signal, and UV light sensing.

Here, we developed a simple method for obtaining a heterojunction composed of graphene (G) and surfactant-coated Au nanoparticles (NPs) to measure film conductivity and surface enhanced Raman scattering (SERS). Monolayer G is obtained by chemical vapor deposition (CVD) and transferred via poly(methyl methacrylate) (PMMA) to microfabricated Au electrodes, glass, and silicon. Post-synthesis treatments of G with PMMA and ozone (O3) showed 1 and 6 orders of magnitude decrease in film conductivity, respectively. The heterojunction formation with Au NPs had no major effect on G conductivity. In this work is demonstrated that G quenches more than 90% of the combined photoluminescence and fluorescence of Au NPs and Rhodamine B (RhB), respectively. Signal quenching permitted quantitative analysis of SERS of RhB on various substrates including as-transferred graphene, oxidized graphene (OG), and the heterojunction. While G is mainly responsible for quenching photoluminescence and fluorescence, ∼3 orders of magnitude increase SERS activity for RhB was accomplished by the heterojunction. Finally, we wanted to correlate changes in film current during UV light sensing experiments. We found striking differences in the sensing profiles at different UV energies.

Liquid-liquid microextraction based on a dispersion of Pd nanoparticles combined with ETAAS for sensitive Hg determination in water samples.

A novel and highly efficient microextraction methodology based on the use of palladium nanoparticles (Pd NPs) was developed for the preconcentration and determination of Hg in water samples. Selective separation of the analyte was achieved by application of dodecanethiolate-coated Pd monolayer-protected clusters (C12S Pd MPCs) in a liquid-liquid microextraction technique (LLME). A volume of 20 µL of toluene phase containing C12S Pd MPCs was used for extraction and final phase was injected in an electrothermal atomic absorption spectrometer (ETAAS) for Hg detection. The effects of different variables, such as sample volume, extraction time, and NPs dispersion volume were carefully studied. A sensitivity enchancement factor of 95 was obtained under optimal experimental conditions. Furthermore, low detection limit (7.5 ng L(-1)) and good precision (relative standard deviation of 4.1% at 0.25 µg L(-1) Hg and n=10) were achieved. The proposed method can be considered as a rapid, cost-effective, and efficient alternative for Hg determination in water samples like river, lake, mineral and tap water.

New findings for the composition and structure of Ni nanoparticles protected with organomercaptan molecules.

Here we explore the synthesis of alkanethiol-coated Ni NPs following the one-phase reaction method by Brust et al. The reduction of NiCl2 with NaBH4 in the presence of dodecanethiol (C12SH) yields a complex product that is difficult to identify as illustrated in the figure of merit. We synthesized Ni(II) dodecanethiolate (C12S) (without the addition of NaBH4) for comparison and performed an exhaustive characterization with TEM, HR-TEM, AFM, MFM, XPS, XRD, UV-vis, magnetism, and FT-IR. It is found that the organic coating is not quite a well-organized self-assembled monolayer (SAM) surrounding the Ni cluster as previously reported. XPS and XRD data show slight differences between both syntheses; however, Ni(II) thiolate appears to be more stable than reduced Ni when exposed to ambient air, indicating the propensity of metallic Ni to oxidize. It has been shown that irradiating with TEM electrons over various metal thiolates leads to nanoparticle formation. We irradiated over Ni(II) thiolate and observed no evidence of NP formation whereas irradiating a reduced Ni sample exhibited an ~3.0 nm nanoparticle diameter. Magnetism studies showed a difference between both samples, indicating ferromagnetic character for the reduced Ni sample. According to our results, the product of the synthesis is comprised of ultrasmall metallic clusters embedded in some form of Ni(II) C12S. In this work, we open a discussion of the chemical nature of the core and the shell in the synthesis of Ni NPs protected with organomercaptan molecules.

Improved vapor selectivity and stability of localized surface plasmon resonance with a surfactant-coated Au nanoparticles film.

Here, we report the use of tetraoctylammonium bromide (TOABr)-coated Au nanoparticles (NPs) for the optical sensing of volatile organic compounds (VOCs). We find that the film responded selectively to the presence of polar and nonpolar vapors by changes in the maximum wavelength (λ(max)) toward higher and lower wavelengths, respectively, as determined by UV-visible spectroscopy. We also observed that the organic coating reorganizes when vapors partition into the film indicated by FT-IR and the film contracts in the presence of water indicated by scanning electron microscopy (SEM). In the present sensor, the metallic Au core serves as the plasmonic signal while the organic coating acts as the receptor material providing vapor selectivity and sensor stability. Correlating changes in (λ(max)) with changes in the refractive index (RI) and nanoparticle-to-nanoparticle separation in the film is important both fundamentally and for improving selectivity in localized surface plasmon resonance (LSPR) sensors.

Chemiresistive sensing with chemically modified metal and alloy nanoparticles.

This review describes the use of chemically modified pure and alloyed metal nanoparticles for chemiresistive sensing applications. Chemically modified metal nanoparticles consist of a pure or alloyed metallic core with some type of chemical coating. Researchers have studied the electronic properties of 1D, 2D, and 3D assemblies of chemically modified metal nanoparticles, and even single individual nanoparticles. The interaction with the analyte alters the conductivity of the sensitive material, providing a signal to measure the analyte concentration. This review focuses on chemiresistive sensing of a wide variety of gas- and liquid-phase analytes with metal nanoparticles coated with organothiols, ions, polymers, surfactants, and biomolecules. Different strategies used to incorporate chemically modified nanoparticles into chemiresistive sensing devices are reviewed, focusing on the different types of metal and alloy compositions, coatings, methods of assembly, and analytes (vapors, gases, liquids, biological materials), along with other important factors.

Hydrogen reactivity of palladium nanoparticles coated with mixed monolayers of alkyl thiols and alkyl amines for sensing and catalysis applications.

Palladium monolayer-protected clusters (MPCs) coated with octylamines (C8NH(2)), hexanethiolates (C6S), and mixed monolayers of C8NH(2) and C6S exhibit significantly different reactivities with hydrogen gas, depending on the relative amounts of the two ligands coating the Pd nanoparticle surface, as determined by UV-vis spectroscopy of Pd MPCs in solution and electronic measurements of films of Pd MPCs as a function of exposure time to hydrogen. The average estimated composition of the ~3.0 nm diameter Pd MPCs was Pd(919)(C6S)(192) or Pd(919)(C8NH(2))(177-x)(C6S)(x), where x was varied to be 0, 3, 10, 16, 32, or 81 by the synthesis of pure C8NH(2) Pd MPCs and subsequent liquid-phase place exchange with a varied amount of C6SH. When x = 0-10, the Pd MPCs react strongly with H(2), leading to aggregated particles in solution and large irreversible changes in the morphology of films accompanied by an increase in film conductivity by 2-5 orders of magnitude. Pd(919)(C6S)(192) MPCs are stable against significant aggregation in solution and do not exhibit large film morphology changes, but they are also not highly reactive to H(2), as determined by minimal changes in the optical properties of solutions and the small, irreversible changes in the conductivity of films in the presence of H(2). Finally, when x is 32 and 81, the Pd MPCs are fairly stable, exhibit minimal aggregation or morphology changes, and readily react with H(2) based on the significant, reversible changes in film conductivity in the presence of H(2). Pd MPCs with mixed monolayers have the benefit of high H(2) reactivity while maintaining the structural stability necessary for sensing and catalysis applications.

Electrochemical fabrication of metal/organic/metal junctions for molecular electronics and sensing applications.

A simple electrochemical approach was used for fabricating electrode/metal nanowire/(molecule or polymer)/electrode junctions for sensing or molecular electronics applications. The procedure for fabricating these molecule-based devices involves electropolymerization of phenol or chemisorption of alkanethiols on one set of electrodes (E1) and electrodeposition of Ag metal nano/microwires on a second electrode (E2) which is ∼5 µm away from E1. Under appropriate deposition conditions, Ag nanowires grow from E2 and cross over to E1, forming a E1/(molecule or polymer)/Ag nanowire (NW)/E2 junction. The junction resistance was controlled by (1) electrodepositing polyphenol of varied densities on E1 and (2) assembling alkanethiols of different chain lengths on E1. Ag NWs at high resistance E1/polyphenol/Ag NW/E2 junctions functionalized with Pd monolayer protected clusters (MPCs) responded fast and reversibly to H(2) concentrations as low as 0.11% in a nitrogen carrier gas by a resistance decrease, likely due to volume expansion of the Pd nanoparticles, demonstrating the use of these electrochemically fabricated junctions for gas sensing applications.

Reactivity of hydrogen with solid-state films of alkylamine- and tetraoctylammonium bromide-stabilized Pd, PdAg, and PdAu nanoparticles for sensing and catalysis applications.

Hydrogen gas spontaneously adsorbs to Pd metal as atomic hydrogen and diffuses into the lattice to form PdHx. We previously showed that films of hexanethiolate-coated Pd monolayer-protected clusters (MPCs) do not readily react with H2 due to the strong chemical bonding of the thiolate to the Pd, which inhibits the reaction. Consequently, these films require ozone or heat treatment for reactivity to occur, which is inconvenient for sensing or catalysis applications. In this report, we describe the reactivity between H2 and solid-state films of alkylamine-coated Pd, PdAg (10:1), and PdAu (10:1) MPCs and films of tetraoctylammonium bromide (TOABr)-stabilized Pd and PdAg (10:1) nanoparticles as determined by changes in film conductivity. Our data show that Pd nanoparticles coated with these more weakly coordinated amine or ammonium groups readily react with H2 without any need for ozone or heat treatment. The conductivity of films of octylamine (C8NH2)- or dodecylamine (C12NH2)-coated Pd, PdAg, and PdAu MPCs increases irreversibly upon initial exposure to 100% H2 to varying degrees and with different reaction kinetics and then exhibits stable, reversible changes in the presence of H2 concentrations ranging from 9.6 to 0.08%. The behavior upon initial exposure to H2 (conditioning) and the direction and magnitude of the reversible conductivity changes depend on the alkyl chainlength and alloy composition. Films of TOABr-coated Pd and PdAg nanoparticles show stable, reversible increases in conductivity in the presence of H2 concentrations from 9.6 down to 0.11% without conditioning. Surface FTIR spectroscopy and atomic force microscopy (AFM) provide information about the organic monolayer and film morphology, respectively, following reactivity with H2. This work demonstrates a simple approach toward preparing films of chemically synthesized Pd-containing nanoparticles with controlled reactivity to H2 for sensing and catalysis applications.

Chemiresistive sensing of volatile organic compounds with films of surfactant-stabilized gold and gold-silver alloy nanoparticles.

Here we describe the chemiresistive sensing of volatile organic compounds (VOCs) with films of chemically synthesized approximately 4 nm diameter Au and AuAg alloy nanoparticles (NPs) stabilized by a surfactant, tetraoctylammonium bromide (TOABr). The chemiresistive sensing properties were measured over a concentration range of 100 to 0.04% saturation for methanol (MeOH), ethanol (EtOH), 2-propanol (IPA), and toluene (Tol) vapor analytes and compared directly to the chemiresistive sensing properties of films of 1.6 nm diameter hexanethiolate (C6S)-coated Au monolayer-protected clusters (MPCs). Films of TOABr-stabilized Au NPs exhibit the opposite response compared to those of C6S-coated Au MPCs. The details are unclear, but the mechanism likely involves changes in capacitive charging in the film or improved conductive pathways through the Au NPs upon incorporation of VOCs into the film for the former as opposed to the well-known change in electron hopping conductivity for the latter. This leads to a decrease in resistance in the presence of VOCs for TOABr Au as opposed to an increase for C6S Au. The TOABr Au sensors are more sensitive, especially for polar analytes, and have greater long-term stability compared to C6S Au. The limit of detection (LOD) for films of TOABr-coated Au NPs is 3, 2, 12, and 37 ppm for IPA, MeOH, EtOH, and Tol, respectively, as compared to 106, 326, 242, and 48 for C6S Au. Films of TOABr-stabilized AuAg alloy NPs exhibit the same type of response, but the sensitivity decreases dramatically with increasing Ag content, showing that the metal composition of the NPs in the film plays a role in the sensing properties, which has not been well-recognized in the literature.

Ozone- and thermally activated films of palladium monolayer-protected clusters for chemiresistive hydrogen sensing.

Here we describe the chemiresistive H2-sensing properties of drop-cast films comprised of 3.0 nm average diameter hexanethiolate-coated Pd monolayer-protected clusters (C6 Pd MPCs) bridging a pair of electrodes separated by a 23 microm gap. The gas-sensing properties were measured for 9.6-0.11% H2 in a H2/N2 mixture. The sensing mechanism is based on changes in the resistance of the film upon reaction of Pd with H2 to form PdH(x), which is known to be larger in volume and more resistive than pure Pd. As-prepared Pd MPC films are highly insensitive to H2, requiring O3 and thermal treatment to enhance changes in film resistance in the presence of H2. Exposure to O3 for 15 min followed by activation in 100% H2 leads to an increase in film conductivity in the presence of H2, with a detection limit of 0.11% H2. When exposed to temperatures of 180-200 degrees C, the conductivity of the film increases and a decrease in conductivity occurs in the presence of H2 with a detection limit of 0.21%. The sensing behavior reverses after further heating to 260 degrees C, exhibiting an increase in conductivity in the presence of H2 as in O3-treated films and a detection limit of 0.11%. The sensitivity of the variously treated films follows the order O3 > high temp > low temp, and the response times at 1.0% H2 range from 10 to 50s, depending on the treatment. FTIR spectroscopy, Raman spectroscopy, and atomic force microscopy provide information about the C6 monolayer, Pd metal, and film morphology, respectively, as a function of O3 and heat treatment to aid in understanding the observed sensing behavior. This work demonstrates a simple chemical approach toward fabricating a fast, reversible sensor capable of detecting low concentrations of H2.