Methylene blue incorporation into alkanethiol SAMs on Au(111): effect of hydrocarbon chain ordering

Unveiling the Nature of lignin's Interaction with Molecules: A Mechanistic Understanding of Adsorption of Methylene Blue Dye

The valorization of lignin into advanced materials for water and soil remediation is experiencing a surge in demand. However, it is imperative that material research and manufacturing be sustainable to prevent exacerbating environmental issues. Meeting these requirements necessitates a deeper understanding of the role of lignin's functional groups in attracting targeted species. This research delves into the interaction mechanisms between lignin and organic molecules, using the adsorption of the cationic dye Methylene Blue (MB+) as a case study. Herein, we aim to quantitatively estimate the contribution of different interaction types to the overall adsorption process. While carbonyl groups were found to have no significant role in attraction, carboxylic groups (-COOH) exhibited significantly lower adsorption compared with hydroxyl groups (-OH). Through alternately blocking aliphatic and phenolic -OH groups, we determined that 61% of the adsorption occurred through hydrogen bonding and 38% via electrostatic interactions. Performance studies of modified lignin along with spectroscopic methods (XPS, FTIR) confirmed the negligible role of π-π interactions in adsorption. This study offers fundamental insights into the mechanistic aspects of MB adsorption on lignin, laying the groundwork for potential modifications to enhance the performance of lignin-based adsorbents. The findings underscore the importance of hydroxyl groups and provide a roadmap for future studies examining the influence of steric factors and interactions with other organic molecules.

Mechanochemical Solid-State Immobilization of Photofunctional Dyes on Amorphous Silica Particles and Investigation of Their Interactive Mechanisms

Amorphous silica particles (ASPs) have been reported to exhibit bioactive properties and are becoming the focus of attention as bioceramics. However, their interactions with proteins in living organisms remain to be understood and need to be investigated in order to achieve wider applications. Our research group found that chlorine (Cl)-containing ASPs are useful for protein immobilization. Photofunctional dyes (fluorescein (FS-), methylene blue (MB+)) that have the carboxy and amino groups as the main functional groups were immobilized on the Cl-containing ASPs via the mechanochemical method as the model molecule and their spectral properties were used to investigate and discuss the organic/inorganic interfacial bonding states. In FS-, the oxygen atoms of the carboxy groups in the molecule were immobilized by the hydrogen bonds with the silanol groups on the ASPs surfaces, indicating that there is an optimum Cl content for the immobilization as the monomer state. In the case of MB+, as the Cl concentration in the ASPs increases, the immobilization via the electrostatic interactions between the Cl in the ASPs and the terminal dimethylamino group, and the hydrogen bonding between the N atoms of the MB+ hetero ring and the particle silanol group were enhanced. These results mainly suggest that the protein adsorption system occurs through the hydrogen bonding between the carboxy groups of the protein and the silanol groups on the particles and via electrostatic interactions between the amino groups of the protein and the dissociated silanol groups and the contained Cl at the particles. Thus, the spectral characterization using dyes as probes is expected to predict the protein interactions with the amorphous silica particles.

Methylene blue@silver nanoprisms conjugates as a strategy against Candida albicans isolated from balanoposthitis using photodynamic inactivation

Balanoposthitis can affect men in immunocompromised situations, such as HIV infection and diabetes. The main associated microorganism is Candida albicans, which can cause local lesions, such as the development of skin cracks associated with itching. As an alternative to conventional treatment, there is a growing interest in the photodynamic inactivation (PDI). It has been shown that the association of photosensitizers with metallic nanoparticles may improve the effectiveness of PDI via plasmonic effect. We have recently shown that the association of methylene blue (MB), a very known photosensitizer, with silver prismatic nanoplatelets (AgNPrs) improved PDI of a resistant strain of Staphylococcus aureus. To further investigate the experimental conditions involved in PDI improvement, in the present study, we studied the effect of MB concentration associated with AgNPrs exploring spectral analysis, zeta potential measurements, and biological assays, testing the conjugated system against C. albicans isolated from a resistant strain of balanoposthitis. The AgNPrs were synthesized through silver anisotropic seed growth induced by the anionic stabilizing agent poly(sodium 4-styrenesulfonate) and showed a plasmon band fully overlapping the MB absorption band. MB and AgNPrs were conjugated through electrostatic association and three different MB concentrations were tested in the nanosystems. Inactivation using red LED light (660 nm) showed a dose dependency in respect to the MB concentration in the conjugates. Using the highest MB concentration (100 µmol⋅L-1) with AgNPr, it was possible to completely inactivate the microorganisms upon a 2 min irradiation exposure. Analyzing optical changes in the conjugates we suggest that these results indicate that AgNPrs are enhancers of MB photodynamic action probably by a combined mechanism of plasmonic effect and reduction of MB dimerization. Therefore, MBAgNPrs can be considered a suitable choice to be applied in PDI of resistant microorganisms.

Developing Prussian blue/wood-derived biochar catalyst for persistent organic pollutant degradation: Preparation, characterization, and mechanism

Biomass-derived biochar shows broad promise for persistent organic pollutants (POPs) degradation and thus establishes a more sustainable homestead. However, effective catalytic performance is still challenging. Herein, an efficient catalyst (Prussian blue decorated wood-derived biochar, PBB) was constructed by introducing Prussian blue (PB) into wood-based biochar to activate peroxymonosulfate (PMS) for removing POPs. After anchoring of PB, the degradation performance of biochar was enhanced (degradation efficiency of methylene blue (MB, 20 mg/L) increased from 52% of biochar to 95% of PBB within 60 min). The PBB presents effective MB degradation performance with a wide pH value (3.0 < pH < 11.0) or co-existing diverse anions (Cl-, NO3-, H2PO4-, and HCO3-). Electron paramagnetic resonance (EPR) analysis as well as electrochemical tests confirmed that the non-radical pathway (1O2) is the key to biochar activation of PMS, but by restricting PB into the biochar, the radical pathway (SO4•- and •OH), the non-radical pathway (1O2), and direct electron transfer can work together to activate PMS. In addition, the degradation efficiency could remain about 80% after five-time cyclic tests. This work elucidates the role of PB nanoparticles in enhancing biochar catalysts, which can inspire the development of a carbon-neutralized, cost-effective, and effective strategy for POPs removal.

Solid waste collagen-associated fabrication of magnetic hematite nanoparticle@collagen nanobiocomposite for emission-adsorption of dyes

The strategic utilization of hazardous particulate waste in eliminating environmental pollution is an important research hotspot. Herein, abundantly available hazardous solid collagenic waste of leather industry is converted into stable hybrid nanobiocomposite (HNP@SWDC) comprising magnetic hematite nanoparticles (HNP) and solid waste derived collagen (SWDC) via co-precipitation method. The structural, spectroscopic, surface, thermal, and magnetic properties; fluorescence quenching; dye selectivity; and adsorption are explored via microstructural analyzes of HNP@SWDC and dye adsorbed-HNP@SWDC using ¹H nuclear magnetic resonance, Raman, ultraviolet-visible, Fourier-transform infrared (FTIR), X-ray photoelectron, and fluorescence spectroscopies; thermogravimetry; field-emission scanning electron microscopy; and vibrating-sample magnetometry (VSM). The intimate interaction of SWDC with HNP and elevated magnetic properties of HNP@SWDC are apprehended via amide-imidol tautomerism associated nonconventional hydrogen bondings, disappearance of goethite specific -OH def. in HNP@SWDC, and VSM. The as-fabricated reusable HNP@SWDC is employed for removing methylene blue (MB) and rhodamine B (RhB). Chemisorption of RhB/MB in HNP@SWDC via ionic, electrostatic, and hydrogen bonding interactions alongside dimerization of dyes are realized by ultraviolet-visible, FTIR, and fluorescence studies; pseudosecond order fitting; and activation energies. The adsorption capacity = 46.98-56.14/22.89-27.57 mg g^-1 for RhB/MB is noted using 0.01 g HNP@SWDC within 5-20 ppm dyes and 288-318 K.

Effective Immobilization of Monomeric Methylene Blue on Hydroxyapatite Nanoparticles by Controlling Inorganic-Organic Interfacial Interactions

We successfully synthesized methylene blue (MB⁺)-immobilized hydroxyapatite (HM) nanoparticles by changing the initial P/Ca molar ratio. The immobilized amount of MB⁺ increased with increasing the initial P/Ca molar ratio from 0.6 to 4.0, and the HM had an elliptical shape (long length, 21-24 nm; short length, 11-13 nm) irrespective of the initial P/Ca molar ratio. Upon increasing the initial P/Ca molar ratio, the number of carbonate ions on the HM surface decreased, which would be owing to the electrostatic repulsion by the surface phosphate ions (i.e., P-O^-), the surface P-OH mainly dissociated to form P-O^-, and the electrostatic interaction of P-O^- with MB⁺ enhanced. The bonding of MB⁺ with surface P-OH and Ca²⁺ sites of hydroxyapatite would be hydrogen-bonding and Lewis acid-base interactions, respectively. The optimum synthesis condition for MB⁺ immobilization at the monomer state was found to be the initial P/Ca molar ratio of 2.0. Here, the existence percentage of the MB⁺ monomer and the molecular occupancy of the surface carbonate ion at the initial P/Ca molar ratio of 2.0 were higher than those at 4.0 with no significant difference in the immobilized amount of MB⁺, indicating that MB⁺ at the initial P/Ca molar ratio of 4.0 is more aggregated than that at 2.0. These results suggested that a part of carbonate ions has a role as a spacer to suppress MB⁺ aggregation. Accordingly, the interfacial interactions between the MB⁺ monomer and the hydroxyapatite surface were clarified, which can effectively be controlled by the initial P/Ca molar ratio. These findings will provide fundamental and useful knowledge for the design of calcium phosphate-organic nanohybrids. We believe that these particles will be the base materials to realize diagnostic and/or therapeutic functions through the molecular state control by optimizing the synthesis conditions.

Enhanced Water Purification Performance of Ionic Liquid Impregnated Metal-Organic Framework: Dye Removal by [BMIM][PF6]/MIL-53(Al) Composite

We incorporated a water-stable ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF₆], into a water-stable metal-organic framework (MOF), MIL-53(Al), to generate the [BMIM][PF₆]/MIL-53(Al) composite. This composite was examined for water purification by studying its capacity for methylene blue (MB) and methyl orange (MO) removal from aqueous solutions having either single dye or a mixture of both. Data illustrated that the removal efficiency and the maximum adsorption capacity of MIL-53(Al) were increased several times upon [BMIM][PF₆] incorporation. For instance, within 1 min, 10 mg of pristine MIL-53(Al) adsorbed 23.3% MB from 10 mg/L of MB solution, while [BMIM][PF₆]/MIL-53(Al) composite was adsorbed 82.3% MB in an identical solution. In the case of MO, 10 mg of pristine MIL-53(Al) achieved 27.8 and 53.6% MO removal from 10 mg/L of MO solution, while [BMIM][PF₆]/MIL-53(Al) composite removed 61.4 and 99.2% within 5 min and 3 h, respectively. Moreover, upon [BMIM][PF₆] incorporation, the maximum MB and MO adsorption capacities of the pristine MOF were increased from 84.5 to 44 mg/g to 204.9 to 60 mg/g, respectively. The adsorption of dyes in pristine MIL-53(Al) and [BMIM][PF₆]/MIL-53(Al) followed a pseudo-second-order kinetic model and a Langmuir isotherm model. In a mixture of both dyes, the IL/MOF composite showed a doubled MB selectivity after the IL incorporation. The composite was successfully regenerated at least two times after its use in water purification to remove MB, MO, and their mixtures. Infrared (IR) spectra indicated that the MB/MO adsorption occurs on [BMIM][PF₆]/MIL-53(Al) by electrostatic interactions, hydrogen bonding, and π-π interactions. These results showed that [BMIM][PF₆]/MIL-53(Al) composite is a highly promising material for efficient water purification.

Molecular and electronic structure of osmium complexes confined to Au(111) surfaces using a self-assembled molecular bridge

The molecular and electronic structure of Os(II) complexes covalently bonded to self-assembled monolayers (SAMs) on Au(111) surfaces was studied by means of polarization modulation infrared reflection absorption spectroscopy, photoelectron spectroscopies, scanning tunneling microscopy, scanning tunneling spectroscopy, and density functional theory calculations. Attachment of the Os complex to the SAM proceeds via an amide covalent bond with the SAM alkyl chain 40° tilted with respect to the surface normal and a total thickness of 26 Å. The highest occupied molecular orbital of the Os complex is mainly based on the Os(II) center located 2.2 eV below the Fermi edge and the LUMO molecular orbital is mainly based on the bipyridine ligands located 1.5 eV above the Fermi edge.

New insight into the electrochemical desorption of alkanethiol SAMs on gold

A combination of Polarization Modulation Infrared Reflection Absorption Spectroscopy (PMIRRAS) under electrochemical control, Electrochemical Scanning Tunneling Microscopy (ECSTM) and Molecular Dynamics (MD) simulations has been used to shed light on the reductive desorption process of dodecanethiol (C12) and octadecanethiol (C18) SAMs on gold in aqueous electrolytes. Experimental PMIRRAS, ECSTM and MD simulations data for C12 desorption are consistent with formation of randomly distributed micellar aggregates stabilized by Na(+) ions, coexisting with a lying-down phase of molecules. The analysis of pit and Au island coverage before and after desorption is consistent with the thiolate-Au adatoms models. On the other hand, PMIRRAS and MD data for C18 indicate that the desorbed alkanethiolates adopt a Na(+) ion-stabilized bilayer of interdigitated alkanethiolates, with no evidence of lying down molecules. MD simulations also show that both the degree of order and tilt angle of the desorbed alkanethiolates change with the surface charge on the metal, going from bilayers to micelles. These results demonstrate the complexity of the alkanethiol desorption in the presence of water and the fact that chain length and counterions play a key role in a complex structure.

The chemistry of the sulfur-gold interface: in search of a unified model

Over the last three decades, self-assembled molecular films on solid surfaces have attracted widespread interest as an intellectual and technological challenge to chemists, physicists, materials scientists, and biologists. A variety of technological applications of nanotechnology rely on the possibility of controlling topological, chemical, and functional features at the molecular level. Self-assembled monolayers (SAMs) composed of chemisorbed species represent fundamental building blocks for creating complex structures by a bottom-up approach. These materials take advantage of the flexibility of organic and supramolecular chemistry to generate synthetic surfaces with well-defined chemical and physical properties. These films already serve as structural or functional parts of sensors, biosensors, drug-delivery systems, molecular electronic devices, protecting capping for nanostructures, and coatings for corrosion protection and tribological applications. Thiol SAMs on gold are the most popular molecular films because the resulting oxide-free, clean, flat surfaces can be easily modified both in the gas phase and in liquid media under ambient conditions. In particular, researchers have extensively studied SAMs on Au(111) because they serve as model systems to understand the basic aspects of the self-assembly of organic molecules on well-defined metal surfaces. Also, great interest has arisen in the surface structure of thiol-capped gold nanoparticles (AuNPs) because of simple synthesis methods that produce highly monodisperse particles with controllable size and a high surface/volume ratio. These features make AuNPs very attractive for technological applications in fields ranging from medicine to heterogeneous catalysis. In many applications, the structure and chemistry of the sulfur-gold interface become crucial since they control the system properties. Therefore, many researchers have focused on understanding of the nature of this interface on both planar and nanoparticle thiol-covered surfaces. However, despite the considerable theoretical and experimental efforts made using various sophisticated techniques, the structure and chemical composition of the sulfur-gold interface at the atomic level remains elusive. In particular, the search for a unified model of the chemistry of the S-Au interface illustrates the difficulty of determining the surface chemistry at the nanoscale. This Account provides a state-of-the-art analysis of this problem and raises some questions that deserve further investigation.

Rapid, sensitive, and quantitative detection of pathogenic DNA at the point of care through microfluidic electrochemical quantitative loop-mediated isothermal amplification

Single-step DNA detection: a microfluidic electrochemical loop mediated isothermal amplification platform is reported for rapid, sensitive, and quantitative detection of pathogen genomic DNA at the point of care. DNA amplification was electrochemically monitored in real time within a monolithic microfluidic device, thus enabling the detection of as few as 16 copies of Salmonella genomic DNA through a single-step process in less than an hour.

Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces

Heptamethinecyanine J-aggregates display sharp, intense fluorescence emission making them attractive candidates for developing a variety of chem-bio-sensing applications. They have been immobilized on planar thiol-covered Au surfaces and thiol-capped Au nanoparticles by weak molecular interactions. In this work the self-assembly of novel thiolated cyanine (CNN) on Au(111) and citrate-capped AuNPs from solutions containing monomers and J-aggregates has been studied by using STM, XPS, PM-IRRAS, electrochemical techniques and Raman spectroscopy. Data show that CNN species adsorb on the Au surfaces by forming thiolate-Au bonds. We found that the J-aggregates are preferentially adsorbed on the Au(111) surface directly from the solution while adsorbed CNN monomers cannot organize into aggregates on the substrate surface. These results indicate that the CNN-Au interaction is not able to disorganize the large J-aggregates stabilized by π-π stacking to optimize the S-Au binding site but it is strong enough to hinder the π-π stacking when CNNs are chemisorbed as monomers. The optical properties of the J-aggregates remain active after adsorption. The possibility of covalently bonding CNN J-aggregates to Au planar surfaces and Au nanoparticles controlling the J-aggregate/Au distance opens a new path regarding their improved stability and the wide range of biological applications of both CNN and AuNP biocompatible systems.

Ligand-controlled rates of photoinduced electron transfer in hybrid CdSe nanocrystal/poly(viologen) films

This paper describes a study of the rates of photoinduced electron transfer (PET) from CdSe quantum dots (QDs) to poly(viologen) within thin films, as a function of the length of the ligands passivating the QDs. Ultrafast (<10 ps), quantitative PET occurs from CdSe QDs coated with HS-(CH(2))(n)-COOH for n = 1, 2, 5, and 7 to viologen units. The observed decrease in the magnitude of the PET rate constant with n is weaker than that expected from the decay of the electron tunneling probability across extended all-trans mercaptocarboxylic acids but well-described by electron tunneling across a collapsed ligand shell. The PET rate constants for films with n = 10 and 15 are much slower than those expected based on the trend for n = 1-7; this deviation is ascribed to the formation of bundles of ligands on the surface of the QD that make the tunneling process prohibitively slow by limiting access of the viologen units to the surfaces of the QDs. This study highlights the importance of molecular-level morphology of donor and acceptor materials in determining the rate and yield of interfacial photoinduced electron transfer in thin films.

Self-assembled monolayers of disulfide Cu porphyrins on Au surfaces: adsorption induced reduction and demetalation

Metalloporphyrin molecules have a wide range of potential applications in diverse technological areas ranging from electronics to optoelectronics, electrochemistry, photophysics, chemical sensors, and catalysis. In particular, self-assembled monolayers of porphyrin molecules have recently attracted considerable interest. In this work we have studied for the first time the self-assembly of a novel Cu deutero porphyrin functionalized with disulfide moieties using electrochemical techniques, UV-vis absorption spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and photoelectron spectroscopies (XPS and UPS). Experimental results indicate that the molecule adsorbs retaining its molecular integrity without forming molecular aggregates via the formation of Au-S covalent bonds. Furthermore, the monolayer consists of a packed array of molecules adsorbed with the plane of the porphyrin molecule at an angle of around 30° with respect to the surface normal. Interestingly, adsorption induces reduction of the Cu center and its consequent removal from the center of the porphyrin ring resulting in porphyrin demetalation. Our results are important in the design of self-assembled monolayers of metallo porphyrins where not only blocking of the metal center by the functional groups that drive the self-assembly should be considered but also possible adsorption induced demetalation with the consequent loss in the properties imparted by the metal center.