Nanoporous Gold Disks Functionalized with Stabilized G‑Quadruplex Moieties for Sensing Small Molecules

We report label-free small molecule sensing on nanoporous gold disks functionalized with stabilized Guanine-quadruplex (G4) moieties using surface-enhanced Raman spectroscopy (SERS). By utilizing the unique G4 topological structure, target molecules can be selectively captured onto nanoporous gold (NPG) disk surfaces via π–π stacking and electrostatic attractions. Together with high-density plasmonic “hot spots” of NPG disks, the captured molecules produce a remarkable SERS signal. Our strategy represents the first example of the detection of foreign molecules conjugated to nondouble helical DNA nanostructures using SERS while providing a new technique for studying the formation and evolution of G4 moieties. The molecular specificity of G4 is known to be controlled by its unit sequence. Without losing generality, we have selected d­(GGT)₇GG sequence for the sensing of malachite green (MG), a known carcinogen frequently abused illegally in aquaculture. The newly developed technique achieved a lowest detectable concentration at an impressive 50 pM, two orders of magnitude lower than the European Union (EU) regulatory requirement, with high specificity against potential interferents. To demonstrate the translational potential of this technology, we achieved a lowest detectable concentration of 5.0 nM, meeting the EU regulatory requirement, using a portable probe based detection system

Fibrillar Self-Organization of a Line-Active Partially Fluorinated Thiol within Binary Self-Assembled Monolayers

Self-assembled monolayers (SAMs) were prepared from a novel two-tailed partially fluorinated thiol (<b>F8C11/C16</b>), possessing one hydrocarbon chain and one chain with an extended fluorinated segment, and from mixtures of <b>F8C11/C16</b> and hexadecanethiol (<b>C16</b>) on gold, with the expectation that the internal chemical dissimilarity and wedge-like shape of <b>F8C11/C16</b> would lead to unique self-organizational motifs. The SAMs were systematically characterized using ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS). Based on this characterization, the one-component <b>F8C11/C16</b> SAMs exhibited relatively poor molecular organization compared to traditional alkanethiols, forming low coverage monolayers with significant molecular disorder. However, the series of mixed SAMs formed from <b>F8C11</b> and <b>F8C11/C16</b> were anomalously well ordered as indicated by film thickness, surface coverage, and the frequencies of characteristic vibrational modes. AFM images of these mixed SAMs exhibited nanoscale fibrillar structures in a birds-nest morphology, suggesting that in the presence of a <b>C16</b> matrix, the <b>F8C11/C16</b> component organized into the two-dimensional analogue of discrete bilayers. Control experiments involving mixed SAMs comprised of <b>F8C11/C16</b> and a single-tailed partially fluorinated thiol (<b>F8C11</b>) or <b>C16</b> and <b>F8C11</b> exhibited no appreciable indication of interesting self-organization beyond an evenly dispersed mixing of the thiolates or phase separation, respectively

Bidentate Aromatic Thiols on Gold: New Insight Regarding the Influence of Branching on the Structure, Packing, Wetting, and Stability of Self-Assembled Monolayers on Gold Surfaces

A series of 2-phenylpropane-1,3-dithiol derivatives with single (<b>R1ArDT</b>), double (<b>R2ArDT</b>), and triple (<b>R3ArDT</b>) octadecyloxy chains substituted at the 4-, 3,5-, and, 3,4,5-positions, respectively, on the aromatic ring were synthesized and used to form self-assembled monolayers (SAMs) on gold. Insight into the relationship between the surface chain and headgroup packing densities was investigated by varying the number of surface chains for the bidentate adsorbates in these monolayers. Characterization of the resulting SAMs using ellipsometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection–absorption spectroscopy, and contact angle goniometry revealed that the tailgroups become more comformationally ordered and more densely packed as the number of alkyl chains per adsorbate was increased. Conversely, the molecular packing density (i.e., number of molecules per unit area) decreased as the number of alkyl chains per adsorbate was increased. Of particular interest, the desorption profiles obtained in isooctane at 80 °C suggested that the bidentate adsorbate with the most densely packed alkyl chains, <b>R3ArDT</b>, was significantly more stable than the other SAMs, producing the following relative order for thermal stability for the dithiolate SAMs: <b>R3ArDT</b> > <b>R2ArDT</b> > <b>R1ArDT</b>

Inverted Surface Dipoles in Fluorinated Self-Assembled Monolayers

The presence of surface dipoles in self-assembled monolayers (SAMs) gives rise to profound effects on the interfacial properties of the films. For example, CF₃-terminated alkanethiolate films are surprisingly more wettable toward polar contacting liquids than analogous hydrocarbon SAMs due to the fluorocarbon-to-hydrocarbon transition (CF₃–CH₂) at the interface (i.e., the presence of a strong “FC–HC” surface dipole). This report explores the converse situation by analyzing partially fluorinated monolayers (FSAMs) in which the polarity of the surface dipole has been inverted through the creation of an “HC–FC” surface dipole. Thus, a new series of methyl-capped partially fluorinated alkanethiols, CH₃(CF₂)₆(CH₂)_<i>n</i>SH (where <i>n</i> = 10–13), were designed and synthesized. Structural analyses of the new films show that these adsorbates give rise to well-ordered monolayers. As for the wetting behavior of various liquids on these FSAMs, the new films proved to be less hydrophobic than both the corresponding CF₃-terminated and hydrocarbon SAMs and more oleophobic than their hydrocarbon counterparts. Furthermore, odd–even trends were observed in the wettability of the nonpolar and polar aprotic liquids on the new films in which the <i>even</i> FSAMs were more wettable than the <i>odd</i> ones for both types of liquids. However, an inverse odd–even trend was observed for polar protic liquids: <i>odd</i> FSAMs were more wettable than <i>even</i>. We attribute this latter effect to the resistance of highly hydrogen-bonded liquid molecules at the liquid–FSAM interface to adopt a more favorable orientation (on the basis of polarity) when in the presence of the inverted HC–FC dipole

DNA Loading and Release Using Custom-Tailored Poly(l‑lysine) Surfaces

This Article describes the generation and study of surfaces modified with custom-crafted poly­(l-lysine) (PLL) coatings for use in the loading and delivery of single-stranded DNA (ssDNA). The experimental strategy utilizes bidentate dithiol adsorbates to generate stably bound azide-terminated self-assembled monolayers (SAMs) on gold possessing an oligo­(ethylene glycol) (OEG) spacer. Consequent to the molecular assembly on gold, the azide termini are covalently attached to a maleimide linker moiety via a copper-catalyzed azide–alkyne “click” reaction. Functionalization with maleimide provides a platform for the subsequent attachment of cysteine-terminated poly­(l-lysine) (PLL), thus forming a suitable surface for the loading of ssDNA via electrostatic interactions. In efforts to maximize DNA loading, we generate SAMs containing mixtures of short and long PLL segments and explore the DNA-loading capability of the various PLL SAMs. We then use thermal increases to trigger the release of the ssDNA from the surface. By examining the loading and release of ssDNA using these new two-dimensional systems, we gain preliminary insight into the potential efficacy of this approach when using three-dimensional gold nanostructure systems in future gene-delivery and biosensing applications

Contrasting Transport and Electrostatic Properties of Selectively Fluorinated Alkanethiol Monolayers with Embedded Dipoles

Surface dipoles are a powerful tool in interfacial modification for improving device output via energy level matching. Fluorinated alkanethiols show a strong promise for these applications as they can generate large and tunable dipoles based on fluorine location and chain length. Furthermore, these chains can be designed to possess fluorocarbons solely along the backbone, enabling an “embedded” configuration that generates a significant dipole effect from the fluorines while maintaining surface chemistry to prevent deleterious side effects from altered surface interactions. However, fluorine substitution can modify other molecular electronic properties, and it is important to consider the transport properties of these interfacial modifiers so that knowledge can be used to tailor the optimal device performance. In this paper, we report the transport properties of self-assembled monolayers derived from a series of fluorinated alkanethiols, both with and without the embedded dipole structure. Photoelectron spectroscopy and Kelvin probe force microscopy show significant work function modification from all fluorine-containing molecules compared to purely hydrocarbon thiols. However, although embedded fluorocarbons generate a smaller electrostatic effect than terminal fluorocarbons, they yield higher tunneling currents across Au/monolayer/eutectic gallium–indium junctions compared to both terminal fluorocarbon and purely hydrocarbon alkanethiols. Computational studies show that the location of the fluorine constituents modifies not only dipoles and energy levels but also molecular orbitals, enabling the presence of delocalized lowest unoccupied molecular orbital levels within the alkanethiol backbone and, thereby, the appearance of larger tunneling currents compared to other alkanethiols. Ultimately, we show that fluorinated alkanethiols and the embedded dipole architecture are both powerful tools, but they must be thoroughly analyzed for proper utilization in a device setting

Magnetic Sensing Potential of Fe₃O₄ Nanocubes Exceeds That of Fe₃O₄ Nanospheres

This paper highlights the relation between the shape of iron oxide (Fe₃O₄) particles and their magnetic sensing ability. We synthesized Fe₃O₄ nanocubes and nanospheres having tunable sizes via solvothermal and thermal decomposition synthesis reactions, respectively, to obtain samples in which the volumes and body diagonals/diameters were equivalent. Vibrating sample magnetometry (VSM) data showed that the saturation magnetization (<i>M</i>_s) and coercivity of 100–225 nm cubic magnetic nanoparticles (MNPs) were, respectively, 1.4–3.0 and 1.1–8.4 times those of spherical MNPs on a same-volume and same-body diagonal/diameter basis. The Curie temperature for the cubic Fe₃O₄ MNPs for each size was also higher than that of the corresponding spherical MNPs; furthermore, the cubic Fe₃O₄ MNPs were more crystalline than the corresponding spherical MNPs. For applications relying on both higher contact area and enhanced magnetic properties, higher-<i>M</i>_s Fe₃O₄ nanocubes offer distinct advantages over Fe₃O₄ nanospheres of the same-volume or same-body diagonal/diameter. We evaluated the sensing potential of our synthesized MNPs using giant magnetoresistive (GMR) sensing and force-induced remnant magnetization spectroscopy (FIRMS). Preliminary data obtained by GMR sensing confirmed that the nanocubes exhibited a distinct sensitivity advantage over the nanospheres. Similarly, FIRMS data showed that when subjected to the same force at the same initial concentration, a greater number of nanocubes remained bound to the sensor surface because of higher surface contact area. Because greater binding and higher <i>M</i>_s translate to stronger signal and better analytical sensitivity, nanocubes are an attractive alternative to nanospheres in sensing applications