Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction.

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force.

Homoleptic complexes of isocyano- and diisocyanobiazulenes with a 12-electron, ligand-based redox capacity.

Oligo- and polyazulenes are attractive π-conjugated building blocks in designing advanced functional materials. Herein, we demonstrate that anchoring one or both isocyanide termini of the redox non-innocent 2,2'-diisocyano-6,6'-biazulenic π-linker (1) to the redox-active [Cr(CO)5] moiety provided a convenient intramolecular redox reference for unambiguously establishing that the 6,6'-biazulenic scaffold undergoes a reversible one-step 2e- reduction governed by reduction potential compression/inversion. Treatment of bis(η6-naphthalene)chromium(0) with six equiv. of 2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene (6) or [(OC)5Cr(η1-2,2'-diisocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (11) afforded homoleptic Cr(0) complexes 13 and 14 with a 12e- (per molecule) ligand-based reduction capacity at mild E1/2 of -1.29 V and -1.15 V vs. Cp2Fe0/+, respectively. The overall reversible redox capacity varies from 15e- for the mononuclear complex 13 to 21e- for the heptanuclear complex 14. The latter "nanocomplex" has a diameter of ca. 5 nm and features seven Cr(0) centers interlinked with six 2,2'-diisocyano-6,6'-biazulenic bridges. The X-ray structure of [(OC)5Cr(2-isocyano-1,1',3,3'-tetraethoxycarbonyl-6,6'-biazulene)] (7) indicated a 43.5° interplanar angle between the two azulenic moieties. Self-assembly of 11 on a Au(111) substrate afforded an organometallic monolayer film of 11 featuring approximately upright orientation of the 2,2'-diisocyano-6,6'-biazulenic linkers, as evidenced by ellipsometric measurements and the RAIR signature of the C4v-symmetric [(-NC)Cr(CO)5] infrared reporter within 11. Remarkably, comparing the FTIR spectrum of 11 in solution with the RAIR spectrum of 11 adsorbed on Au(111) suggested electronic coupling at a ca. 2 nm distance between the Cr(0) and Au atoms linked by the 2,2'-diisocyano-6,6'-biazulene bridge.

MoS2 Nanodonuts for High-Sensitivity Surface-Enhanced Raman Spectroscopy.

Nanohybrids of graphene and two-dimensional (2D) layered transition metal dichalcogenides (TMD) nanostructures can provide a promising substrate for extraordinary surface-enhanced Raman spectroscopy (SERS) due to the combined electromagnetic enhancement on TMD nanostructures via localized surface plasmonic resonance (LSPR) and chemical enhancement on graphene. In these nanohybrid SERS substrates, the LSPR on TMD nanostructures is affected by the TMD morphology. Herein, we report the first successful growth of MoS2 nanodonuts (N-donuts) on graphene using a vapor transport process on graphene. Using Rhodamine 6G (R6G) as a probe, SERS spectra were compared on MoS2 N-donuts/graphene nanohybrids substrates. A remarkably high R6G SERS sensitivity up to 2 × 10-12 M has been obtained, which can be attributed to the more robust LSPR effect than in other TMD nanostructures such as nanodiscs as suggested by the finite-difference time-domain simulation. This result demonstrates that non-metallic TMD/graphene nanohybrids substrates can have SERS sensitivity up to one order of magnitude higher than that reported on the plasmonic metal nanostructures/2D materials SERS substrates, providing a promising scheme for high-sensitivity, low-cost applications for biosensing.

Probing Selective Self-Assembly of Putrescine Oxidase with Controlled Orientation Using a Genetically Engineered Peptide Tag.

Controlling enzyme orientation and location on surfaces is a critical step for their successful deployment in diverse applications from biosensors to lab-on-a-chip devices. Functional activity of the enzymes on the surface will largely depend on the spatial arrangement and orientation. Solid binding peptides have been proven to offer versatility for immobilization of biomolecules on inorganic materials including metals, oxides, and minerals. Previously, we demonstrated the utility of a gold binding peptide genetically incorporated into the enzyme putrescine oxidase (PutOx-AuBP), enabling self-enzyme assembly on gold substrates. PutOx is an attractive biocatalyst among flavin oxidases, using molecular oxygen as an electron acceptor without requiring a dissociable coenzyme. Here, we explore the selective self-assembly of this enzyme on a range of surfaces using atomic force microscopy (AFM) along with the assessment of functional activity. This work probes the differences in surface coverage, distribution, size, shape, and activity of PutOx-AuBP in comparison to those of native putrescine oxidase (PutOx) on multiple surfaces to provide insight for material-selective enzymatic assembly. Surfaces investigated include metal (templated-stripped gold (TSG)), oxide (native SiO2 on Si(111)), minerals (mica and graphite), and self-assembled monolayers (SAMs) with a range of hydrophobicity and charge. Supported by both the coverage and the dimensions of immobilized enzymes, our results indicate that of the surfaces investigated, material-selective binding takes place with orientation control only for PutOx-AuBP onto the TSG substrate. These differences are consistent with the measurements of surface-bound enzymatic activities. Substrate-dependent differences observed indicate significant variations in enzyme-surface interactions ranging from peptide-directed self-assembly to enzyme aggregation. The implications of this study provide insight for the fabrication of enzymatic patterns directed by self-assembling peptide tags onto localized surface regions. Enabling functional enzyme-based nanoscale materials offers a fascinating path for utilization of sustainable biocatalysts integrated into multiscale devices.

Development of an ALD-Pt@SWCNT/Graphene 3D Nanohybrid Architecture for Hydrogen Sensing.

A nanohybrid architecture composed of single-wall carbon nanotube films and graphene heterostructures (SWCNT/graphene) was developed as a three-dimensional (3D) electrode. Atomic layer deposition (ALD) was used for conformal coating of catalytic Pt nanoparticles on the 3D ALD-Pt@SWCNT/graphene nanohybrid architecture for further enhancement of H2 sensing, taking advantage of the large sensing area and conformally coated nanostructures of the catalytic Pt. Remarkably, the H2 response was found to be improved by 50% in the SWCNT/graphene nanohybrid, indicating that graphene provides a more efficient charge transport. The ALD-Pt further enhances the H2 responsivity of the 3D ALD-Pt @SWCNT/graphene nanohybrids. By coating 10 cycles of ALD-Pt on the SWCNT/graphene nanohybrid, the H2 response (2.77%) is approximately twice that (1.4%) of its counterpart without the ALD-Pt. By further optimizing the 3D ALD-Pt@SWCNT/graphene nanohybrids with respect to the ALD-Pt cycle numbers and SWCNT film thickness, a H2 responsivity as high as 7.5% was achieved on the SWCNT/graphene nanohybrid sample with a 560 nm thick SWCNT film and 50 cycles of ALD-Pt.

Self-Immobilized Putrescine Oxidase Biocatalyst System Engineered with a Metal Binding Peptide.

Flavin oxidases are valuable biocatalysts for the oxidative synthesis of a wide range of compounds, while at the same time reduce oxygen to hydrogen peroxide. Compared to other redox enzymes, their ability to use molecular oxygen as an electron acceptor offers a relatively simple system that does not require a dissociable coenzyme. As such, they are attractive targets for adaptation as cost-effective biosensor elements. Their functional immobilization on surfaces offers unique opportunities to expand their utilization for a wide range of applications. Genetically engineered peptides have been demonstrated as enablers of the functional assembly of biomolecules at solid material interfaces. Once identified as having a high affinity for the material of interest, these peptides can provide a single step bioassembly process with orientation control, a critical parameter for functional immobilization of the enzymes. In this study, for the first time, we explored the bioassembly of a putrescine oxidase enzyme using a gold binding peptide tag. The enzyme was genetically engineered to incorporate a gold binding peptide with an expectation of an effective display of the peptide tag to interact with the gold surface. In this work, the functional activity and expression were investigated, along with the selectivity of the binding of the peptide-tagged enzyme. The fusion enzyme was characterized using multiple techniques, including protein electrophoresis, enzyme activity, and microscopy and spectroscopic methods, to verify the functional expression of the tagged protein with near-native activity. Binding studies using quartz crystal microbalance (QCM), nanoparticle binding studies, and atomic force microscopy studies were used to address the selectivity of the binding through the peptide tag. Surface binding AFM studies show that the binding was selective for gold. Quartz crystal microbalance studies show a strong increase in the affinity of the peptide-tagged protein over the native enzyme, while activity assays of protein bound to nanoparticles provide evidence that the enzyme retained catalytic activity when immobilized. In addition to showing selectivity, AFM images show significant differences in the height of the molecules when immobilized through the peptide tag compared to immobilization of the native enzyme, indicating differences in orientation of the bound enzyme when attached via the affinity tag. Controlling the orientation of surface-immobilized enzymes would further improve their enzymatic activity and impact diverse applications, including oxidative biocatalysis, biosensors, biochips, and biofuel production.

Mechanistic investigations of matrix metalloproteinase-8 inhibition by metal abstraction peptide.

The mechanism of matrix metalloproteinase-8 (MMP-8) inhibition was investigated using ellipsometric measurements of the interaction of MMP-8 with a surface bound peptide inhibitor, tether-metal abstraction peptide (MAP), bound to self-assembled monolayer films. MMP-8 is a collagenase whose activity and dysregulation have been implicated in a number of disease states, including cancer metastasis, diabetic neuropathy, and degradation of biomedical reconstructions, including dental restorations. Regulation of activity of MMP-8 and other matrix metalloproteinases is thus a significant, but challenging, therapeutic target. Strong inhibition of MMP-8 activity has recently been achieved via the small metal binding peptide tether-MAP. Here, the authors elucidate the mechanism of this inhibition and demonstrate that it occurs through the direct interaction of the MAP Tag and the Zn(2+) binding site in the MMP-8 active site. This enhanced understanding of the mechanism of inhibition will allow the design of more potent inhibitors as well as assays important for monitoring critical MMP levels in disease states.

First π-linker featuring mercapto and isocyano anchoring groups within the same molecule: Synthesis, heterobimetallic complexation and self-assembly on Au(111).

Mercapto (-SH) and isocyano (-N≡C) terminated conducting π-linkers are often employed in the ever-growing quest for organoelectronic materials. While such systems typically involve symmetric dimercapto or diisocyano anchoring of the organic bridge, this article introduces the chemistry of a linear azulenic π-linker equipped with one mercapto and one isocyano terminus. The 2-isocyano-6-mercaptoazulene platform was efficiently accessed from 2-amino-6-bromo-1,3-diethoxycarbonylazulene in four steps. The 2-N≡C end of this 2,6-azulenic motif was anchrored to the [Cr(CO)5] fragment prior to formation of its 6-SH terminus. Metalation of the 6-SH end of [(OC)5Cr(η1-2-isocyano-1,3-diethoxycarbonyl-6-mercaptoazulene)] (7) with Ph3PAuCl, under basic conditions, afforded X-ray structurally characterized heterobimetallic Cr0/AuI ensemble [(OC)5Cr(µ-η1:η1-2-isocyano-1,3-diethoxycarbonyl-6-azulenylthiolate)AuPPh3] (8). Analysis of the 13C NMR chemical shifts for the [(NC)Cr(CO)5] core in a series of the related complexes [(OC)5Cr(2-isocyano-6-X-1,3-diethoxy-carbonylazulene)] (X = -N≡C, Br,H, SH, SCH2CH2CO2CH2CH3, SAuPPh3) unveiled remarkably consistent inverse-linear correlations δ( 13COtrans) vs. δ( 13CN) and δ( 13COcis) vs. δ( 13CN) that appear to hold well beyond the above 2-isocyanoazulenic series to include complexes [(OC)5Cr(CNR)] containing strongly electron-withdrawing substituents R, such as CF3, CFClCF2Cl, C2F3, and C6F5. In addition to functioning as asensitive 13C NMR handle, the essentially C4v-symmetric [(-NC)Cr(CO)5] moiety proved to be an informative, remote, νN≡C/νC≡O infrared reporter in probing chemisorption of 7 on the Au(111) surface.

Development of methacrylate/silorane hybrid monomer system: Relationship between photopolymerization behavior and dynamic mechanical properties.

Resin chemistries for dental composite are evolving as noted by the introduction of silorane-based composites in 2007. This shift in the landscape from methacrylate-based composites has fueled the quest for versatile methacrylate-silorane adhesives. The objective of this study was to evaluate the polymerization behavior and structure/property relationships of methacrylate-silorane hybrid systems. Amine compound ethyl-4-(dimethylamino) benzoate (EDMAB) or silane compound tris(trimethylsilyl) silane (TTMSS) was selected as coinitiators. The mechanical properties of the copolymer were improved significantly at low concentrations (15, 25, or 35 wt %) of silorane when EDMAB was used as coinitiator. The rubbery moduli of these experimental copolymers were increased by up to 260%, compared with that of the control (30.8 ± 1.9 MPa). Visible phase separation appeared in these formulations if the silorane concentrations in the formulations were 50-75 wt %. The use of TTMSS as coinitiator decreased the phase separation, but there was a concomitant decrease in mechanical properties. In the neat methacrylate formulations, the maximum rates of free-radical polymerization with EDMAB or TTMSS were 0.28 or 0.06 s(-1) , respectively. In the neat silorane resin, the maximum rates of cationic ring-opening polymerization with EDMAB or TTMSS were 0.056 or 0.087 s(-1) , respectively. The phase separation phenomenon may be attributed to differences in the rates of free-radical polymerization of methacrylates and cationic ring-opening polymerization of silorane. In the hybrid systems, free-radical polymerization initiated with EDMAB led to higher crosslink density and better mechanical properties under dry/wet conditions. These beneficial effects were, however, associated with an increase in heterogeneity in the network structure. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 841-852, 2016.

Grafting MAP peptide to dental polymer inhibits MMP-8 activity.

Matrix metalloproteinases (MMPs) are a class of zinc and calcium-dependent endopeptidases responsible for degrading extracellular matrix (ECM) components. Their activity is critical for both normal biological function and pathological processes (Dejonckheere et al., Cytokine Growth Factor Rev 2011;22:73-81). In dental restorations, the release and subsequent acid activation of MMPs contributes to premature failure. In particular, MMP-8 accelerates degradation by cleaving the collagen matrix within the dentin substrate in incompletely infiltrated aged bonded dentin (Buzalaf et al., Adv Dent Res 2012;24:72-76), hastening the need for replacement of restorations. Therefore, development of a dental adhesive that better resists MMP-8 activity is of significant interest. We hypothesize that modification of the polymer surface with an inhibitor would disable MMP-8 activity. Here, we identify the metal abstraction peptide (MAP) as an inhibitor of MMP-8 and demonstrate that tethering MAP to methacrylate polymers effectively inhibits catalysis. Our findings indicate complete inhibition of MMP-8 is achievable using a grafting approach. This strategy has potential to improve longevity of dental adhesives and other polymers and enable rational design of a new generation of biocompatible materials.

Ancillary nitrile substituents as convenient IR spectroscopic reporters for self-assembly of mercapto- and isocyanoazulenes on Au(111).

Synthesis and self-assembly of structurally related mercapto- and isocyanoazulenes, including novel 2-mercapto-1,3-dicyanoazulene (4) and 2-isocyano-1,3-dicyanoazulene (5), are reported. Exposing 5 adsorbed on Au(111) to a solution of 4 displaces the isocyanoazulene monolayer with that of the mercaptoazulene as judged by ν(C≡N) signatures of these films.

Linear 6,6'-biazulenyl framework featuring isocyanide termini: synthesis, structure, redox behavior, complexation, and self-assembly on Au(111).

The key step in accessing the title species (5), the first nonbenzenoid diisocyanobiaryl, involved an unexpected homocoupling of a 6-bromoazulene derivative. The reversible 2e(-) reduction of 5 was addressed electrochemically and computationally. The shifts in energies of the S(0)→S(1) and S(0)→S(2) transitions for a series of related 6,6'-biazulenyl derivatives correlate with the e(-)-donating/-withdrawing strength of their 2,2'-substituents but follow opposite trends. Species 5 adsorbs end-on (η(1)) to the Au(111) surface via one of its -NC groups to form a 2-nm-thick film. In addition, bimetallic coordination of 5's -NC termini can be readily achieved.

Multipoint anchoring of the [2.2.2.2]metacyclophane motif to a gold surface via self-assembly: coordination chemistry of a cyclic tetraisocyanide revisited.

A one-pot transformation of bis(2-isocyano-3-methylphenyl)ethane affords gram quantities of 8,16,24,32-tetraisocyano[2.2.2.2]metacyclophane ( 3). The solid state structure of 3 is remarkably close to the lowest energy conformation found on the potential energy landscape for 3 by DFT. In solution, the structure of metacyclophane 3 is mobile but can be locked in a rectangular gauche- anti- gauche- anti conformation by coordination of the isocyanide substituents to the [W(CO) 5] units to give [M] 4(mu 4-eta (1):eta (1):eta (1):eta (1)- 3) ( 5). The tetranuclear [M] 4(mu 4-eta (1):eta (1):eta (1):eta (1)- 3) motif featured in crystallographically characterized 5 may be present in several insoluble complexes of 3 previously described as mononuclear eta (4) species. A self-assembled monolayer of metacyclophane 3 is formed upon exposing a solution of 3 to the gold(111) surface with no precautions to exclude air or light. The monolayer nature of the film was confirmed by optical ellipsometry. The isocyanide stretching band for 3 shifts from 2119 cm (-1) in solution to 2175 cm (-1) upon chemisorption to metallic gold. The FTIR spectrum of the film indicates interaction of 3 with the gold surface via all four of its isocyanide anchors. No gold-facilitated oxidation of the -NC junctions was detected under ambient conditions. The energy cost associated with accessing the conformations of 3 suitable for mu 4-eta (1):eta (1):eta (1):eta (1) interaction of the molecule with the Au(111) surface is under 8 kcal/mol, a value that can be easily offset by formation of a gold-isocyanide bond. Two different mu 4-eta (1):eta (1):eta (1):eta (1) coordination arrangements of 3 with respect to gold atoms on the (111) face of the fcc Au lattice are suggested.

Mutations within the C-terminus of the gamma subunit of the photosynthetic F1-ATPase activate MgATP hydrolysis and attenuate the stimulatory oxyanion effect.

Two highly conserved amino acid residues near the C-terminus within the gamma subunit of the mitochondrial ATP synthase form a "catch" with an anionic loop on one of the three beta subunits within the catalytic alphabeta hexamer of the F1 segment [Abrahams, J. P., Leslie, A. G. W., Lutter, R., and Walker, J. E. (1994) Nature 370, 621-628]. Forming the catch is considered to be an essential step in cooperative nucleotide binding leading to gamma subunit rotation. The analogous residues, Arg304 and Gln305, in the chloroplast F1 gamma subunit were changed to leucine and alanine, respectively. Each mutant gamma was assembled together with alpha and beta subunits from Rhodospirillum rubrum F1 into a hybrid photosynthetic F1 that carries out both MgATPase and CaATPase activities and ATP-dependent gamma rotation [Tucker, W. C., Schwarcz, A., Levine, T., Du, Z., Gromet-Elhanan, Z., Richter, M. L. and Haran, G. (2004) J. Biol. Chem. 279, 47415-47418]. Surprisingly, changing Arg304 to leucine resulted in a more than 2-fold increase in the kcat for MgATP hydrolysis. In contrast, changing Gln305 to alanine had little effect on the kcat but completely abolished the well-known stimulatory effect of the oxyanion sulfite on MgATP hydrolysis. The MgATPase activities of combined mutants with both residues substituted were strongly inhibited, whereas the CaATPase activities were inhibited, but to a lesser extent. The results indicate that the C-terminus of the photosynthetic F1 gamma subunit, like its mitochondrial counterpart, forms a catch with the alpha and beta subunits that modulates the nucleotide binding properties of the catalytic site(s). The catch is likely to be part of an activation mechanism, overcoming inhibition by free mg2+ ions, but is not essential for cooperative nucleotide exchange.

Structural analysis of the regulatory dithiol-containing domain of the chloroplast ATP synthase gamma subunit.

The gamma subunit of the F1 portion of the chloroplast ATP synthase contains a critically placed dithiol that provides a redox switch converting the enzyme from a latent to an active ATPase. The switch prevents depletion of intracellular ATP pools in the dark when photophosphorylation is inactive. The dithiol is located in a special regulatory segment of about 40 amino acids that is absent from the gamma subunits of the eubacterial and mitochondrial enzymes. Site-directed mutagenesis was used to probe the relationship between the structure of the gamma regulatory segment and its function in ATPase regulation via its interaction with the inhibitory epsilon subunit. Mutations were designed using a homology model of the chloroplast gamma subunit based on the analogous structures of the bacterial and mitochondrial homologues. The mutations included (a) substituting both of the disulfide-forming cysteines (Cys199 and Cys205) for alanines, (b) deleting nine residues containing the dithiol, (c) deleting the region distal to the dithiol (residues 224-240), and (d) deleting the entire segment between residues 196 and 241 with the exception of a small spacer element, and (e) deleting pieces from a small loop segment predicted by the model to interact with the dithiol domain. Deletions within the dithiol domain and within parts of the loop segment resulted in loss of redox control of the ATPase activity of the F1 enzyme. Deleting the distal segment, the whole regulatory domain, or parts of the loop segment had the additional effect of reducing the maximum extent of inhibition obtained upon adding the epsilon subunit but did not abolish epsilon binding. The results suggest a mechanism by which the gamma and epsilon subunits interact with each other to induce the latent state of the enzyme.

Interaction of mono- and diisocyanoazulenes with gold surfaces: first examples of self-assembled monolayer films involving azulenic scaffolds.

The formation and properties of a new class of self-assembled monolayers (SAM) of aryl isocyanides and diisocyanides based on the nonbenzenoid azulenic framework have been investigated using FTIR spectroscopy and ellipsometry. Syntheses of several new members of the isocyanoazulene family, a recently established type of aryl isocyanides, are reported as well. The FTIR spectra for the isocyanoazulene derivatives absorbed on the gold surface indicate the terminal upright coordination of every isocyanoazulene molecule studied. In addition, the ellipsometric thicknesses have been measured and are consistent with those calculated for single monolayers of the isocyanides oriented along the surface normal. Unlike SAMs of some benzenoid aryl isocyanides, the nonbenzenoid isocyanoazule-based SAMs proved resistant to oxidation, oligomerization, and isomerization into the corresponding nitriles under ambient conditions, which is an important prerequisite to their future applications.

Nanoscale patterning of alkyl monolayers on silicon using the atomic force microscope.

Self-assembled monolayers (SAMs) of 1-alkenes on hydrogen-passivated silicon substrates were successfully patterned on the nanometer scale using an atomic force microscope (AFM) probe tip. Nanoshaving experiments on alkyl monolayers formed on H-Si(111) not only demonstrate the flexibility of this technique but also show that patterning with an AFM probe is a viable method for creating well-defined, nanoscale features in a monolayer matrix in a reproducible and controlled manner. Features of varying depths (2-15 nm) were created in the alkyl monolayers by controlling the applied load and the number of etching scans made at high applied loads. The patterning on these SAM films is compared with the patterning of alkyl siloxane monolayers on silicon and mica.

Alternative method for fabricating chemically functionalized AFM tips: silane modification of HF-treated Si3N4 probes.

An alternative method for fabricating functionalized, atomic force microscopy (AFM) tips is presented. This technique is simple and requires only minimal preparation and tip modification to generate chemically sensitive probes that have a robust organic monolayer of flexible terminal chemistry attached to the surface. Specifically, commercially microfabricated Si3N4 AFM tips were modified with self-assembled monolayers (SAMs) of octadecyltrichlorosilane and (11-bromoundecyl)trichlorosilane after removing the native silicon oxide surface layer with concentrated hydrofluoric acid. The structure of these SAM films on solid silicon nitride surfaces was studied using contact angle goniometry and Fourier transform infrared spectroscopy. Pull-off force measurements on various bare (mica, graphite, and silicon) and SAM-functionalized substrates confirm that mechanically robust, long-chain organic silane SAMs can be formed on HF-treated Si3N4 tips without the presence of an intervening oxide layer. Adhesion experiments show that the integrity of the organic film on the chemically modified tips is maintained over repeated measurements and that the functionalized tips can be used for chemical sensing experiments since strong discrimination between different surface chemistries is possible.