Metal-Organic Frameworks in Surface Enhanced Raman Spectroscopy-Based Analysis of Volatile Organic Compounds.

Volatile Organic Compounds (VOC) are a major class of environmental pollutants hazardous to human health, but also highly relevant in other fields including early disease diagnostics and organoleptic perception of aliments. Therefore, accurate analysis of VOC is essential, and a need for new analytical methods is witnessed for rapid on-site detection without complex sample preparation. Surface-Enhanced Raman Spectroscopy (SERS) offers a rapidly developing versatile analytical platform for the portable detection of chemical species. Nonetheless, the need for efficient docking of target analytes at the metallic surface significantly narrows the applicability of SERS. This limitation can be circumvented by interfacing the sensor surface with Metal-Organic Frameworks (MOF). These materials featuring chemical and structural versatility can efficiently pre-concentrate low molecular weight species such as VOC through their ordered porous structure. This review presents recent trends in the development of MOF-based SERS substrates with a focus on elucidating respective design rules for maximizing analytical performance. An overview of the status of the detection of harmful VOC is discussed in the context of industrial and environmental monitoring. In addition, a survey of the analysis of VOC biomarkers for medical diagnosis and emerging applications in aroma and flavor profiling is included.

Unified Roadmap for ZIF-8 Nucleation and Growth: Machine Learning Analysis of Synthetic Variables and Their Impact on Particle Size and Morphology.

Metal-organic frameworks (MOFs) have settled in the scientific community over the last decades as versatile materials with several applications. Among those, zeolitic imidazolate framework 8 (ZIF-8) is a well-known MOF that has been applied in various and diverse fields, from drug-delivery platforms to microelectronics. However, the complex role played by the reaction parameters in controlling the size and morphology of ZIF-8 particles is still not fully understood. Even further, many individual reports propose different nucleation and growth mechanisms for ZIF-8, thus creating a fragmented view for the behavior of the system. To provide a unified view, we have generated a comprehensive data set of synthetic conditions and their final outputs and applied machine learning techniques to analyze the data. Our approach has enabled us to identify the nucleation and growth mechanisms operating for ZIF-8 in a given sub-space of synthetic variables space (chemical space) and to reveal their impact on important features such as final particle size and morphology. By doing so, we draw connections and establish a hierarchy for the role of each synthetic variable and provide with rule of thumb for attaining control on the final particle size. Our results provide a unified roadmap for the nucleation and growth mechanisms of ZIF-8 in agreement with mainstream reported trends, which can guide the rational design of ZIF-8 particles which ultimately determine their suitability for any given targeted application. Altogether, our work represents a step forward in seeking control of the properties of MOFs through a deeper understanding of the rationale behind the synthesis procedures employed for their synthesis.

Switchable Ion Current Saturation Regimes Enabled via Heterostructured Nanofluidic Devices Based on Metal-Organic Frameworks.

The use of track-etched membranes allows further fine-tuning of transport regimes and thus enables their use in (bio)sensing and energy-harvesting applications, among others. Recently, metal-organic frameworks (MOFs) have been combined with such membranes to further increase their potential. Herein, the creation of a single track-etched nanochannel modified with the UiO-66 MOF is proposed. By the interfacial growth method, UiO-66-confined synthesis fills the nanochannel completely and smoothly, yet its constructional porosity renders a heterostructure along the axial coordinate of the channel. The MOF heterostructure confers notorious changes in the transport regime of the nanofluidic device. In particular, the tortuosity provided by the micro- and mesostructure of UiO-66 added to its charged state leads to iontronic outputs characterized by an asymmetric ion current saturation for transmembrane voltages exceeding 0.3 V. Remarkably, this behavior can be easily and reversibly modulated by changing the pH of the media and it can also be maintained for a wide range of KCl concentrations. In addition, it is found that the modified-nanochannel functionality cannot be explained by considering just the intrinsic microporosity of UiO-66, but rather the constructional porosity that arises during the MOF growth process plays a central and dominant role.

Biofunctionalization of Graphene-Based FET Sensors through Heterobifunctional Nanoscaffolds: Technology Validation toward Rapid COVID-19 Diagnostics and Monitoring.

The biofunctionalization of graphene field-effect transistors (GFETs) through vinylsulfonated-polyethyleneimine nanoscaffold is presented for enhanced biosensing of severe acute respiratory-related coronavirus 2 (SARS-CoV-2) spike protein and human ferritin, two targets of great importance for the rapid diagnostic and monitoring of individuals with COVID-19. The heterobifunctional nanoscaffold enables covalent immobilization of binding proteins and antifouling polymers while the whole architecture is attached to graphene by multivalent π-π interactions. First, to optimize the sensing platform, concanavalin A is employed for glycoprotein detection. Then, monoclonal antibodies specific against SARS-CoV-2 spike protein and human ferritin are anchored, yielding biosensors with limit of detections of 0.74 and 0.23 nm, and apparent affinity constants ( K D G F E T ) of 6.7 and 8.8 nm, respectively. Both biosensing platforms show good specificity, fast time response, and wide dynamic range (0.1-100 nm). Moreover, SARS-CoV-2 spike protein is also detected in spiked nasopharyngeal swab samples. To rigorously validate this biosensing technology, the GFET response is matched with surface plasmon resonance measurements, exhibiting linear correlations (from 2 to 100 ng cm-2) and good agreement in terms of K D values. Finally, the performance of the biosensors fabricated through the nanoscaffold strategy is compared with those obtained through the widely employed monopyrene approach, showing enhanced sensitivity.

Surface Engineering of Graphene through Heterobifunctional Supramolecular-Covalent Scaffolds for Rapid COVID-19 Biomarker Detection.

Graphene is a two-dimensional semiconducting material whose application for diagnostics has been a real game-changer in terms of sensitivity and response time, variables of paramount importance to stop the COVID-19 spreading. Nevertheless, strategies for the modification of docking recognition and antifouling elements to obtain covalent-like stability without the disruption of the graphene band structure are still needed. In this work, we conducted surface engineering of graphene through heterofunctional supramolecular-covalent scaffolds based on vinylsulfonated-polyamines (PA-VS). In these scaffolds, one side binds graphene through multivalent π-π interactions with pyrene groups, and the other side presents vinylsulfonated pending groups that can be used for covalent binding. The construction of PA-VS scaffolds was demonstrated by spectroscopic ellipsometry, Raman spectroscopy, and contact angle measurements. The covalent binding of -SH, -NH2, or -OH groups was confirmed, and it evidenced great chemical versatility. After field-effect studies, we found that the PA-VS-based scaffolds do not disrupt the semiconducting properties of graphene. Moreover, the scaffolds were covalently modified with poly(ethylene glycol) (PEG), which improved the resistance to nonspecific proteins by almost 7-fold compared to the widely used PEG-monopyrene approach. The attachment of recognition elements to PA-VS was optimized for concanavalin A (ConA), a model lectin with a high affinity to glycans. Lastly, the platform was implemented for the rapid, sensitive, and regenerable recognition of SARS-CoV-2 spike protein and human ferritin in lab-made samples. Those two are the target molecules of major importance for the rapid detection and monitoring of COVID-19-positive patients. For that purpose, monoclonal antibodies (mAbs) were bound to the scaffolds, resulting in a surface coverage of 436 ± 30 ng/cm2. KD affinity constants of 48.4 and 2.54 nM were obtained by surface plasmon resonance (SPR) spectroscopy for SARS-CoV-2 spike protein and human ferritin binding on these supramolecular scaffolds, respectively.

Growth of ZIF-8 MOF Films with Tunable Porosity by using Poly (1-vinylimidazole) Brushes as 3D Primers.

This work reports on a novel and versatile approach to control the structure of metal-organic framework (MOFs) films by using polymeric brushes as 3D primers, suitable for triggering heterogeneous MOF nucleation. As a proof-of-concept, this work explores the use of poly(1-vinylimidazole) brushes primer obtained via surface-initiated atom transfer radical polymerization (SI-ATRP) for the synthesis of Zn-based ZIF-8 MOF films. By modifying the grafting density of the brushes, smooth porous films were obtained featuring inherently hydrophobic microporosity arising from ZIF-8 structure, and an additional constructional interparticle mesoporosity, which can be employed for differential adsorption of targeted adsorbates. It was found that the grafting density modulates the constructional porosity of the films obtained; higher grafting densities result in more compact structures, while lower grafting density generates increasingly inhomogeneous films with a higher proportion of interparticle constructional porosity.

Triggering doxorubicin release from responsive hydrogel films by polyamine uptake.

Polyamines such as putrescine, spermidine and spermine are required in many inter- and intra-cellular processes. There is, however, evidence of anomalously high concentrations of these polyamines around cancer cells. Furthermore, high polyamine concentrations play a key role in accelerating the speed of cancer proliferation. Some current therapies target the reduction of the polyamine concentration to delay the cancer advance. In this study, we use a molecular theory to prove the concept that poly(methacrylic acid) (PMAA) hydrogels can play the dual role of incorporating and retaining polyamines as well as releasing preloaded drugs in response. Towards such a goal, we have developed a molecular model for each of the chemical species, which includes the shape, size, charge, protonation state, and configuration. Our results indicate that PMAA hydrogel films can incorporate significant amounts of polyamines; this absorption increases with the solution concentration of the polyamines. Doxorubicin was chosen as a model drug for this study, which can be successfully incorporated within the film; the optimal encapsulation conditions occur at low salt concentrations and pH values near neutral. Polyamine absorption within the film results in the desorption of the drug from the hydrogel. An increase in the concentration of the polyamines enhances the drug release. To validate our theoretical findings, poly(methacrylic acid) hydrogel thin films were synthesized by atom transfer radical polymerization. Absorption/desorption experiments followed by UV-Vis spectroscopy demonstrate doxorubicin encapsulation within these films and polyamine-dependent drug release.

Synthesis and characterization of thermoresponsive ZIF-8@PNIPAm-co-MAA microgel composites with enhanced performance as an adsorption/release platform.

Composite materials featuring a synergic combination of interesting properties such as stimuli responsiveness and tailored porosity are highly appealing due to their multiple possible applications. We hereby present an example which brings together such features by using poly(N-isopropyl-acrylamide)-derived thermo-responsive microgels and Zn-based Metal Organic Framework (MOF) ZIF-8, capable of selective adsorption. Such a composite was obtained by including methacrylic acid as a co-monomer in the microgel, in order to position carboxylic acid moieties within the polymeric matrix, which via preconcentration of MOF precursors would trigger confined heterogeneous nucleation. The highly integrated composite obtained features thermoresponsivity and permanent porosity. Methylene blue adsorption/desorption experiments were performed, revealing a dramatic enhancement of its cargo capacity together with an increased release efficiency.

Shedding Light on the Dark Corners of Metal-Organic Framework Thin Films: Growth and Structural Stability of ZIF-8 Layers Probed by Optical Waveguide Spectroscopy.

Metal-organic framework (MOF) thin films are promising materials for multiple technological applications, such as chemical sensing. However, one potential limitation for their widespread use in different settings is their stability in aqueous environments. In the case of ZIF-8 (zeolitic imidazolate framework) thin films, their stability in aqueous media is currently a matter of debate. Here, we show that optical waveguide spectroscopy (OWS), in combination with surface plasmon resonance (SPR) spectroscopy, offers a convenient way for answering intriguing questions related to the stability of MOF thin films in aqueous solutions and, eventually provide a tool for assessing changes in MOF layers under different environmental conditions. Our experiments relied on the use of ZIF-8 thin films grown on surface-modified gold substrates, as optical waveguides. We have found a linear thickness increase after each growing cycle and observed that the growing characteristics are strongly influenced by the nature of the primer layer. One of our findings is that substrate surface modification with a 3-mercapto-1-propanesulfonate (MPSA) primer layer is critical to achieve ZIF-8 layers that can effectively act as optical waveguides. We observed that ZIF-8 films are structurally stable upon exposure to pure water and 50 mM NaCl solutions but they exhibit a slight swelling and an increase in porosity probably due to the permeation of the solvent in the intergrain mesoporous cavities. However, OWS revealed that exposure of ZIF-8 thin films to phosphate-buffered saline solutions (pH 8) promotes significant film degradation. This poses an important question as to the prospective use of ZIF-8 materials in biologically relevant applications. In addition, it was demonstrated that postsynthetic polyelectrolyte modification of ZIF-8 films has no detrimental effects on the structural stability of the films.

Polyelectrolyte Capping As Straightforward Approach toward Manipulation of Diffusive Transport in MOF Films.

We present experimental results demonstrating the suitability of polyelectrolyte capping as a simple and straightforward procedure to modify hydrophilic/hydrophobic character of porous films, thus allowing additional control on transport properties. In particular, we synthesized ZIF-8 metal organic framework (MOF) films, an archetypal hydrophobic zeolitic imidazolate framework, constituted by Zn2+ ions tetrahedrally coordinated with bidentate 2-methylimidazolate organic linker, and poly(4-styrenesulfonic acid) as capping agent (PSS). MOF films were synthesized via sequential one pot (SOP) steps over conductive substrates conveniently modified with primer agents known to enhance heterogeneous nucleation, followed by dip-coating with PSS aqueous solutions. Crystallinity, morphology, and chemical composition of ZIF-8 films were confirmed with traditional methods. Continuous electron density depth profile obtained with synchrotron light X-ray reflectivity (XRR) technique, suggest that PSS capped-films do not adopt segregated configurations in which PSS remains surface-confined. This affects functional properties conferred by PSS capping, which were assessed using cyclic voltammetry with both positively and negatively charged redox probe molecules. Furthermore, taking advantage of the control attained, we successfully carried in situ synthesis of film-hosted d-block metal nanoparticles (Au and Pt-NPT@5x-ZIF-8+PSS) via direct aqueous chemical reduction of precursors (diffusion-reaction approach).