Cavitands Endow All-Dielectric Beads With Selectivity for Plasmon-Free Enhanced Raman Detection of Nε-Methylated Lysine

Structural diversity in the coordination compounds of cobalt, nickel and copper with N-alkylglycinates: crystallographic and ESR study in the solid state

Reactions of N-methylglycine (HMeGly), N-ethylglycine-hydrochloride (H₂EtGlyCl) and N-propylglycine-hydrochloride (H₂PrGlyCl) with cobalt(ii), nickel(ii) and copper(ii) ions in aqueous solutions resulted in ten new coordination compounds [Co(MeGly)₂(H₂O)₂] (1), [{Co(MeGly)₂}₂(μ-OH)₂]·2H₂O (1d), [Cu(MeGly)₂(H₂O)₂] (2α), [Co(EtGly)₂(H₂O)₂] (3), [Ni(EtGly)₂(H₂O)₂] (4), [Cu(μ-EtGly)₂]_n (5p), [Co(PrGly)₂(H₂O)₂] (6), [Ni(PrGly)₂(H₂O)₂] (7), and two polymorphs of [Cu(PrGly)₂(H₂O)₂] (8α and 8β). Compounds were characterized by single-crystal and powder X-ray diffraction, infrared spectroscopy, thermal analysis and X-band electron spin resonance (ESR) spectroscopy. These studies revealed a wide range of structural types including monomeric, dimeric and polymeric architectures, as well as different polymorphs. In all monomeric compounds, except 2α, and in the coordination polymer 5p hydrogen bonds interconnect the molecules into 2D layers with the alkyl chain pointing outward of the layer. In 2α and in the dimeric compound 1d hydrogen bonds link the molecules into 3D structures. 1d with cobalt(iii), and 4 and 7 with nickel(ii) are ESR silent. The ESR spectra of 1, 3 and 6 are characteristic for paramagnetic high-spin cobalt(ii). The ESR spectra of all copper(ii) coordination compounds show that the unpaired copper electron is located in the d_x²−y² orbital, being in agreement with the elongated octahedral geometry.

Interactions in copper, nickel and cobalt complexes with N-methyl-, N-ethyl- and N-propylglycinate: monomers, dimer and polymers.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Biochemical sensing with macrocyclic receptors

Preventive healthcare asks for the development of cheap, precise and non-invasive sensor devices for the early detection of diseases and continuous population screening. The actual techniques used for diagnosis, e.g. MRI and PET, or for biochemical marker sensing, e.g. immunoassays, are not suitable for continuous monitoring since they are expensive and prone to false positive responses. Synthetic supramolecular receptors offer new opportunities for the creation of specific, selective and cheap sensor devices for biological sensing of specific target molecules in complex mixtures of organic substances. The fundamental challenges faced in developing such devices are the precise transfer of the molecular recognition events at the solid-liquid interface and its transduction into a readable signal. In this review we present the progress made so far in turning synthetic macrocyclic hosts, namely cyclodextrins, calixarenes, cucurbiturils and cavitands, into effective biochemical sensors and the strategies utilized to solve the above mentioned issues. The performances of the developed sensing devices based on these receptors in detecting specific biological molecules, drugs and proteins are critically discussed.

Ultrasensitive Surface-Enhanced Raman Spectroscopy Detection Based on Amorphous Molybdenum Oxide Quantum Dots

Surface-enhanced Raman spectroscopy (SERS) based on plasmonic semiconductive material has been proved to be an efficient tool to detect trace of substances, while the relatively weak plasmon resonance compared with noble metal materials restricts its practical application. Herein, for the first time a facile method to fabricate amorphous H_x MoO₃ quantum dots with tunable plasmon resonance is developed by a controlled oxidization route. The as-prepared amorphous H_x MoO₃ quantum dots show tunable plasmon resonance in the region of visible and near-infrared light. Moreover, the tunability induced by SC CO₂ is analyzed by a molecule kinetic theory combined with a molecular thermodynamic model. More importantly, the ultrahigh enhancement factor of amorphous H_x MoO₃ quantum dots detecting on methyl blue can be up to 9.5 × 10⁵ with expending the limit of detection to 10^-9 m. Such a remarkable porperty can also be found in this H_x MoO₃ -based sensor with Rh6G and RhB as probe molecules, suggesting that the amorphous H_x MoO₃ quantum dot is an efficient candidate for SERS on molecule detection in high precision.

Non-Plasmonic SERS with Silicon: Is It Really Safe? New Insights into the Optothermal Properties of Core/Shell Microbeads

Silicon is one of the most interesting candidates for plasmon-free surface-enhaced Raman scattering (SERS), because of its high-refractive index and thermal stability. However, here we demonstrate that the alleged thermal stability of silicon nanoshells irradiated by conventional Raman laser cannot be taken for granted. We investigated the opto-thermal behavior of SiO₂/Si core/shell microbeads (Si-rex) irradiated with three common Raman laser sources (λ = 532, 633, 785 nm) under real working conditions. We obtained an experimental proof of the critical role played by bead size and aggregation in heat and light management, demonstrating that, in the case of strong opto-thermal coupling, the temperature can exceed that of the melting points of both core and shell components. In addition, we also show that weakly coupled beads can be utilized as stable substrates for plasmon-free SERS experiments.

Photo-induced heat generation in non-plasmonic nanoantennas

The photo-induced heat generation in SiO₂/Si core/shell nanoantennas is analysed on the basis of their optothermal properties.

Mechanism of Surface-Enhanced Raman Scattering Based on 3D Graphene-TiO2 Nanocomposites and Application to Real-Time Monitoring of Telomerase Activity in Differentiation of Stem Cells

With a burst development of new nanomaterials for plasmon-free surface-enhanced Raman scattering (SERS), the understanding of chemical mechanism (CM) and further applications have become more and more attractive. Herein, a novel SERS platform was specially designed through electrochemical deposition of graphene onto TiO₂ nanoarrays (EG-TiO₂). The developed EG-TiO₂ nanocomposite SERS platform possessed remarkable Raman activity using copper phthalocyanine (CuPc) as a probe molecule. X-ray photoelectron spectroscopy measurement revealed that the chemical bond Ti-O-C was formed at the interface between graphene and TiO₂ in EG-TiO₂ nanocomposites. Both experimental and theoretical results demonstrated that the obvious Raman enhancement was attributed to TiO₂-induced Fermi level shift of graphene, resulting in effective charge transfer between EG-TiO₂ nanocomposites and molecules. Taking advantage of a marked Raman response of the CuPc molecule on the EG-TiO₂ nanocomposite surface as well as specific recognition of CuPc toward multiple telomeric G-quadruplex, EG-TiO₂ nanocomposites were tactfully employed as the SERS substrate for selective and ultrasensitive determination of telomerase activity, with a low detection limit down to 2.07 × 10^-16 IU. Interestingly, the self-cleaning characteristic of EG-TiO₂ nanocomposites under visible light irradiation successfully provided a recycling ability for this plasmon-free EG-TiO₂ substrate. The present SERS biosensor with high analytical performance, such as high selectivity and sensitivity, has been further explored to determine telomerase activity in stem cells as well as to count the cell numbers. More importantly, using this useful tool, it was discovered that telomerase activity plays an important role in the proliferation and differentiation from human mesenchymal stem cells to neural stem cells. This work has not only established an approach for gaining fundamental insights into the chemical mechanism (CM) of Raman enhancement but also has opened a new way in the investigation of long-term dynamics of stem cell differentiation and clinical drug screening.

ZORRO: zirconium oxide resonators for all-in-one Raman and whispering-gallery-mode optical sensing

We report the observation of whispering-gallery modes in 2 μm-sized SiO₂/ZrO₂ core/shell beads utilized as all-dielectric Raman enhancers. This allows us to achieve simultaneous optical and Raman ultrasensitive detection with a single spectral analysis. This opportunity opens exciting perspectives for the multimodal chemical sensing and fabrication of optical fiber devices.

Laser-induced construction of multi-branched CuS nanodendrites with excellent surface-enhanced Raman scattering spectroscopy in repeated applications

We report on the successful fabrication of multi-branched CuS nanodendrites with average branch length of about 20 nm by laser ablation of bulk Cu target in thioacetamide (TAA) solution. During the nucleation of Cu and S species, the accurate anisotropic growth should be attributed to an ultra-rapid acid etching process by laser-induced TAA hydrolyzing reaction. Interestingly, the semiconductor CuS nanodendrites provide pronounced surface enhanced Raman scattering (SERS) properties with noble-metal comparable activity and a detection limit as low as ~10^-10 M, approaching the requirement (~nM) for single molecule detection. More importantly, after SERS analysis, the crystal violet (CV) probe molecules can be effectively removed from the substrate by 1064nm laser irradiation-induced moderate thermal treatment. Therefore, the unique and distinctive advantage is that the as-prepared CuS nanodendrites exhibit excellent reusability for 60 cycles of repeated SERS analyses. The low-cost CuS semiconductor nanodendrites with enhanced SERS properties should be established as a prominent SERS-based ultrasensitive probe in the repeated applications.

Probing lysine mono-methylation in histone H3 tail peptides with an abiotic receptor coupled to a non-plasmonic resonator

Binder and effector molecules that allow studying and manipulating epigenetic processes are of biological relevance and pose severe technical challenges. We report the first example of a synthetic receptor able to recognize mono-methylated lysines in a histone H3 tail peptide, which has relevant functions in epigenetic regulation. Recognition is robust and specific regardless of the position and the number of mono-methylated lysines along the polypeptide chain. The peptide is first captured in solution by a tetraphosphonate cavitand (Tiiii) that selectively binds its Lys-NMe⁺ moieties. Separation from solution and detection of the peptide-Tiiii complexes is then enabled in one single step by an all dielectric SiO₂-TiO₂ core-shell resonator (T-rex), which captures the complex and operates fully reproducible signal transduction by non-plasmonic surface enhanced Raman scattering (SERS) without degrading the complex. The realized abiotic probe is able to distinguish multiple mono-methylated peptides from the single mono-methylated ones.

Ultrasensitive SERS Detection by Defect Engineering on Single Cu2 O Superstructure Particle

A Cu₂ O superstructure is constructed through a recrystallization-induced self-assembly strategy. Single Cu₂ O superstructure particle exhibits an outstanding surface-enhanced Raman spectroscopy performance with the limit of detection as low as 10^-9 mol L^-1 and metal comparable enhancement factor (8 × 10⁵ ) due to the synergetic effect of vacancies defect-facilitated charge-transfer process and copper vacancies defect-induced electrostatic adsorption.

"RaMassays": Synergistic Enhancement of Plasmon-Free Raman Scattering and Mass Spectrometry for Multimodal Analysis of Small Molecules

SiO₂/TiO₂ core/shell (T-rex) beads were exploited as "all-in-one" building-block materials to create analytical assays that combine plasmon-free surface enhanced Raman scattering (SERS) and surface assisted laser desorption/ionization (SALDI) mass spectrometry (RaMassays). Such a multi-modal approach relies on the unique optical properties of T-rex beads, which are able to harvest and manage light in both UV and Vis range, making ionization and Raman scattering more efficient. RaMassays were successfully applied to the detection of small (molecular weight, M.W. <400 Da) molecules with a key relevance in biochemistry and pharmaceutical analysis. Caffeine and cocaine were utilized as molecular probes to test the combined SERS/SALDI response of RaMassays, showing excellent sensitivity and reproducibility. The differentiation between amphetamine/ephedrine and theophylline/theobromine couples demonstrated the synergistic reciprocal reinforcement of SERS and SALDI. Finally, the conversion of L-tyrosine in L-DOPA was utilized to probe RaMassays as analytical tools for characterizing reaction intermediates without introducing any spurious effects. RaMassays exhibit important advantages over plasmonic nanoparticles in terms of reproducibility, absence of interference and potential integration in multiplexed devices.

Midrefractive Dielectric Modulator for Broadband Unidirectional Scattering and Effective Radiative Tailoring in the Visible Region

Nanoantennas have found many applications in ultrasmall sensors, single-molecule detection, and all-optical communication. Conventional nanoantennas are based on noble-metal plasmonic structures, but suffer from large ohmic loss and only possess dipolar plasmon modes. This has driven an intense search for all-dielectric materials beyond noble metals. Here, we propose midrefractive nanospheres as a novel all-dielectric material to realize broadband unidirectional radiation and effective radiative tailoring in the visible region. Midrefractive all-dielectric materials such as boron nanospheres possess broad and overlapping electric and magnetic dipole modes. The internal interaction between these two modes can route the radiation almost on the one side covering the whole visible range. Unlike the elaborate design in plasmonic nanostructures to obtain strong coupled broad and narrow modes, the bright mode in boron nanospheres is intrinsic, independent, and easily coupled with adjacent narrow modes. So the strong interaction in boron-based heterodimer is able to realize an independent and precise tailoring of the radiant and subradiant states by simply changing the particle sizes, respectively. Our findings imply midrefractivity materials like boron are excellent building blocks to support electromagnetic coupling operation in nanoscale devices, which will lead to a variety of emerging applications such as nanoantennas with directing exciton emission, ultrasensitive nanosensors, or even potential new construction of photonic metamaterials.

The Origin of Selectivity in the Complexation of N-Methyl Amino Acids by Tetraphosphonate Cavitands

We report on the eligibility of tetraphosphonate resorcinarene cavitands for the molecular recognition of amino acids. We determined the crystal structure of 13 complexes of the tetraphosphonate cavitand Tiiii[H, CH3, CH3] with amino acids. (1)H NMR and (31)P NMR experiments and ITC analysis were performed to probe the binding between cavitand Tiiii[C3H7, CH3, C2H5] or the water-soluble counterpart Tiiii[C3H6Py(+)Cl(-), CH3, C2H5] and a selection of representative amino acids. The reported studies and results allowed us (i) to highlight the noncovalent interactions involved in the binding event in each case; (ii) to investigate the ability of tetraphosphonate cavitand receptors to discriminate between the different amino acids; (iii) to calculate the Ka values of the different complexes formed and evaluate the thermodynamic parameters of the complexation process, dissecting the entropic and enthalpic contributions; and (iv) to determine the solvent influence on the complexation selectivity. By moving from methanol to water, the complexation changed from entropy driven to entropy opposed, leading to a drop of almost three orders in the magnitude of the Ka. However, this reduction in binding affinity is associated with a dramatic increase in selectivity, since in aqueous solutions only N-methylated amino acids are effectively recognized. The thermodynamic profile of the binding does not change in PBS solution. The pivotal role played by cation-π interactions is demonstrated by the linear correlation found between the log Ka in methanol solution and the depth of (+)N-CH3 cavity inclusion in the molecular structures. These findings are relevant for the potential use of phosphonate cavitands as synthetic receptors for the detection of epigenetic modifications of histones in physiological media.

A tetrasulfate-resorcin[6]arene cavitand as the host for organic ammonium guests

A resorcin[6]arene cavitand bearing sulfate bridges is herein reported. The host is able to interact with ammonium guests through the sulfate bridges.