Biofunctionalization of Si(111)7×7 by “Renewable” l-Cysteine Transitional Layer

Constructing a Comprehensive Nanopattern Library through Morphological Transitions of Block Copolymer Surface Micelles via Direct Solvent Immersion

This study establishes a comprehensive library of nanopatterns achievable by a single block copolymer (BCP), ranging from spheres to complex structures like split micelles, flower-like clusters, toroids, disordered micelle arrays, and unspecified unique shapes. The ordinary nanostructures of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) surface micelles deposited on a SiOx surface undergo a unique morphology transformation when immersed directly in solvents. Investigating parameters such as immersion solvents, BCP molecular weight, substrate interactions, and temperature, this work reveals the influence of these parameters on the thermodynamics and kinetics governing the morphology transformation. Additionally, the practical application of BCP nanopattern templates for fabricating metal nanostructures through direct solvent immersion of surface micelles is demonstrated. This approach offers an efficient and effective method for producing diverse nanostructures, with the potential to be employed in nanolithography, catalysts, electronics, membranes, plasmonics, and photonics.

Covalent and Hydrogen Bonding in Adsorption of Alanine Molecules on Si(111)7×7

Molecular adsorption bonding configurations and specific interfacial chemistry of alanine on Si(111)7×7 have been determined by combining the results from scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) with ab initio calculations based on the density functional theory (DFT). XPS spectra of the N 1s region show that alanine molecules bind to the 7×7 surface by N-Si covalent bonding, while STM imaging reveals that such N-H dissociative adsorption of alanine occurs on an adjacent Si adatom-restatom pair, with the dehydrogenated alanine moiety and dissociated H atom occupying the Si adatom and restatom sites, respectively. At a sample bias above +2 V, the dehydrogenated alanine appears as a bright round protrusion, slightly off-center from a Si adatom site and leaning toward the opposite Si adatom across the dimer wall. The off-center character can be attributed to an electrostatic attraction between the electron-rich carbonyl O of the dehydrogenated alanine and electron-deficient nearest Si adatom across the dimer wall. Our DFT calculation also shows that the monodentate O-Si bonding configuration resulting from O-H dissociative adsorption is more thermodynamically favorable than the experimentally observed N-Si bonding configuration, suggesting that the interfacial dissociative adsorption reaction is a kinetically controlled rather than a thermodynamically driven process. Alanine molecules in the second adlayer (transitional layer) are found to attach to those in the first adlayer (interfacial layer) by N···HO hydrogen bonding, as supported by the presence of the N 1s feature at 401.0 eV. An alanine molecule H-bonded to a dehydrogenated alanine in the first adlayer has also been observed in STM as a brighter and larger protrusion close to the expected location of the free OH group in the dehydrogenated first-adlayer alanine. No thick zwitterionic alanine film can be obtained at room temperature possibly due to steric constraint caused by the methyl group.

Structural and Chemical Evolution of l-Cysteine Nanofilm on Si(111)-√3×√3-Ag: From Preferential Growth at Step Edges and Antiphase Boundaries at Room Temperature to Adsorbate-Mediated Metal Cluster Formation at Elevated Temperature

The interaction of cysteine molecules with the Si(111)-√3×√3-Ag surface has been investigated over the submonolayer to multilayer regime using X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory calculations. With both upper step and lower step terraces, step edges, and antiphase boundaries, the √3×√3-Ag overlayer supported on Si(111) provides a rich two-dimensional template for studying site-specific biomolecular interactions. As an amino acid with three functional groups, cysteine is found to chemisorb through S-H bond cleavage and S-Ag bond linkage first at step edges and antiphase boundaries followed by island formation and expanded growth onto terraces. Intermolecular interactions are dominated by zwitterionic hydrogen bonding at higher coverages, producing a porous unordered interfacial layer composed of cysteine agglomerates at room temperature. Upon annealing, cysteine adsorbates induce structural transformation of the uniform √3×√3-Ag reconstructed surface lattice into metallic Ag clusters with a narrow size distribution and short-range ordering. Preferential nanoaggregate formation of cysteine at defect sites and cysteine-induced metal cluster formation promise a new approach to fabricating nanoclusters for potential applications in chemical sensing and catalysis.

Self-Assembled Copolymer Adsorption Layer-Induced Block Copolymer Nanostructures in Thin Films

In polymer thin films, the bottom polymer chains are irreversibly adsorbed onto the substrates creating an ultrathin layer. Although this thin layer (only a few nanometers thick) governs all film properties, an understanding of this adsorbed layer remains elusive, and thus, its effective control has yet to be achieved, particularly in block copolymer (BCP) thin films. Herein, we employ self-assembled copolymer adsorption layers (SCALs), transferred from the air/water interfacial self-assembly of BCPs, as an effective control of the adsorbed layer in BCP thin films. SCALs replace the natural adsorbed layer, irreversibly adsorbing onto the substrates when other BCP is additionally coated on the SCALs. We further show that SCALs guide the thin film nanostructures because they provide topological restrictions and enthalpic/entropic preferences for a BCP self-assembly. The SCAL-induced self-assembly enables unprecedented control of nanostructures, creating novel nanopatterns such as spacing-controlled hole/dot patterns, dotted-line patterns, dash-line patterns, and anisotropic cluster patterns with exceptional controllability.

pH-Dependent photoluminescence “switch-on” nanosensors composed of silver nanoparticles and nitrogen and sulphur co-doped carbon dots for discriminative detection of biothiols

A nanomaterial surface energy transfer (NSET) system composed of silver nanoparticles (AgNPs) and nitrogen and sulphur co-doped carbon dots (N,S-CDs) was established to discriminate biothiols, featuring the pH-promoted distinct PL “switch-on” response.

Controlled synthesis of mixed molecular nanostructures from folate and deferrioxamine-Ga(III) on gold and tuning their performance for cancer cells

A new strategy is developed for construction of the mixed molecular nanostructures from folic acid (FOA), a targeting agent, and deferrioxamoine-Ga(III), (DFO-Ga(III)), a theranostic agent, on gold-mercaptopropionic acid surface, Au-MPA. The strategy is focused to achieve a system in which all the active constituents of FOA; i.e., pteridine rings, p-aminobenzoeic acid, and the glutamic acid, having high affinity for folate receptor overexpressed on cancer cells; remain unreacted in adjacent to DFO-Ga(III), Au-MPA-[DFO-Ga(III)]‖-[FOA]. For this purpose, the NH₂ groups of FOA and DFO-Ga(III) were attached covalently and separately to COOH of Au-MPA surface allowing all the active groups of FOA to be available for drug delivery purposes. The data obtained through several electrochemical and surface analysis techniques, supported successful construction of the designed mixed molecular nanostructures system. In addition, the results showed that the system is stable, and Ga(III) ion does not leave DFO-Ga(III) complex. The prepared surface was successfully tested for capturing of the breast cancer cells 4T1 as a model. The measurements showed a rapid uptake kinetics (t_1/2 of ~6.0min) and efficient accessibility of the system by the cancer cells; the R_ct was significantly increased in the presence of 4T1 cells compared with blank PBS (ΔR_ct ~420kΩ).

A photoluminescence “switch-on” nanosensor composed of nitrogen and sulphur co-doped carbon dots and gold nanoparticles for discriminative detection of glutathione

A nanosensor was established to discriminate glutathione (GSH) from other competitive biothiols based on a photoluminescence (PL) “switch-on” signal readout.

Surface-Mediated Hydrogen Bonding of Proteinogenic α-Amino Acids on Silicon

Understanding the adsorption, film growth mechanisms, and hydrogen bonding interactions of biological molecules on semiconductor surfaces has attracted much recent attention because of their applications in biosensors, biocompatible materials, and biomolecule-based electronic devices. One of the most challenging questions when studying the behavior of biomolecules on a metal or semiconductor surface is "What are the driving forces and film growth mechanisms for biomolecular adsorption on these surfaces?" Despite a large volume of work on self-assembly of amino acids on single-crystal metal surfaces, semiconductor surfaces offer more direct surface-mediated interactions and processes with biomolecules. This is due to their directional surface dangling bonds that could significantly perturb hydrogen bonding arrangements. For all the proteinogenic biomolecules studied to date, our group has observed that they generally follow a "universal" three-stage growth process on Si(111)7×7 surface. This is supported by corroborating data obtained from a three-pronged approach of combining chemical-state information provided by X-ray photoelectron spectroscopy (XPS) and the site-specific local density-of-state images obtained by scanning tunneling microscopy (STM) with large-scale quantum mechanical modeling based on the density functional theory with van der Waals corrections (DFT-D2). Indeed, this three-stage growth process on the 7×7 surface has been observed for small benchmark biomolecules, including glycine (the simplest nonchiral amino acid), alanine (the simplest chiral amino acid), cysteine (the smallest amino acid with a thiol group), and glycylglycine (the smallest (di)peptide of glycine). Its universality is further validated here for the other sulfur-containing proteinogenic amino acid, methionine. We use methionine as an example of prototypical proteinogenic amino acids to illustrate this surface-mediated process. This type of growth begins with the formation of a covalent-bond driven interfacial layer (first adlayer), followed by that of a transitional layer driven by interlayer and intralayer hydrogen bonding (second adlayer), and then finally the zwitterionic multilayers (with intralayer hydrogen bonding). The important role of surface-mediated hydrogen bonding as the key for this universal three-stage growth process is demonstrated. This finding provides new insight into biomolecule-semiconductor surface interactions often found in biosensors and biomolecular electronic devices. We also establish the trends in the H-bond length among different types of the hydrogen bonding for dimolecular structures in the gas phase and on the Si(111)7×7 surface, the latter of which could be validated by their STM images. Finally, five simple rules of thumb are developed to summarize the adsorption properties of these proteinogenic biomolecules as mediated by hydrogen bonding, and they are expected to provide a helpful guide to future studies of larger biomolecules and their potential applications.

One-step synthesis of chiral carbon quantum dots and their enantioselective recognition

Chiral carbon quantum dots (l-carbon quantum dots, l-CQDs; and d-carbon quantum dots, d-CQDs) were synthesized through the facile hydrothermal treatment of carbonated citric acid and l-cysteine (or d-cysteine).