Determination of self-exchange rate of alkanethiolates in self-assembled monolayers on gold using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Enhanced synthesis of antibody-functionalized gold nanoparticles for multiplexed exosome detection via mass signal amplification in LDI-TOF MS

We present a novel method for sensitive exosomal protein detection using organic matrix-free laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) and gold nanoparticles (AuNPs) functionalized with mass tags for signal amplification (Am-tags). Target exosomes were captured by specific antibodies on AuNPs and a biochip, where the antibody-presenting AuNPs (Ab/Am-tag@AuNPs) contained excess Am-tags. LDI-TOF MS analysis revealed the mass signal of Am-tags on Ab/Am-tag@AuNPs, indicating the presence of target exosomes. Thus, the target signal was amplified by a large number of Am-tags, resulting in enhanced sensitivity. We optimized the protocol to prepare stable Ab/Am-tag@AuNPs, focusing on parameters such as the concentration and ratio of thiol molecules for AuNP functionalization, suitable solvents for the coupling reaction, and amount of antibodies conjugated to the AuNPs. Subsequently, we evaluated the ability of our method to detect exosomes isolated from three cell lines, NIH3T3, MCF7, and HeLa, using an anti-Rab5 immobilized gold chip and anti-CD63/Am-tag@AuNPs with LDI-TOF MS analysis. Calibration curves constructed for the three cell lines showed a linear relationship with an excellent limit of detection. Finally, we emphasized the versatility of our method for the quantitative detection of exosomal proteins CD63 and mucin 1 (MUC1) using two types of Am-tags. LDI-TOF MS analysis revealed the presence of CD63 and MUC1 at different expression levels in HeLa and MCF7 cancer cells. Our findings clearly indicate the potential of Ab/Am-tag@AuNPs as a sensitive and reliable approach for identifying biomarkers in exosomes, providing valuable insights into their utility in biomedical research and clinical settings.

Controllable release of dopamine from simulated enzyme-containing biomembrane by biased potential

Controllable release of dopamine (DA) is worth studying for its significant in physiological metabolic process. β-cyclodextrin/11-mercaptoundecanoic acid self-assembled monolayer (β-CD/MUA SAM) based on hydrogen bonds network was constructed as simulated enzyme-containing biomembrane. DA interacted with β-CD based on intermolecular hydrogen bond and formed inclusion complexes in SAM, namely DA@β-CD/MUA SAM. The desorption process of DA from DA@β-CD/MUA SAM revealed the release behavior of DA from enzyme-containing biomembrane at molecular level. Positive biased potential as external stimulus was applied at DA@β-CD/MUA SAM, reducing the negative charge density of SAM. The decrease of negative charge density of SAM resulted in the weakening of hydrogen bond between DA and β-CD, which in turn caused DA to be released. Using scanning electrochemical microscopy, positive biased potential not only had specificity induction for the release of DA, but also was able to monitor the release quantitatively in real time. These results showed that positive biased potential as external stimulus was favorable for the controllable release of DA, suggesting the possible application of biased potential in controllable regulation field.

Cooperativity and coverage dependent molecular desorption in self-assembled monolayers: computational case study with coronene on Au(111) and HOPG

One of the common practices in the literature of molecular desorption is the comparison of theoretically (mostly using DFT) calculated single molecule adsorption energies with experimental desorption energies from studies like temperature programmed desorption (TPD) etc. Comparisons like those do not consider that the experimental desorption energies are obtained via ensemble techniques while theoretical values are calculated at the single molecule level. Theoretical values are generally based upon desorption of a single molecule from a clean surface, or upon desorption of an entire monolayer. On the other hand, coverage dependent molecule-molecule interactions add to and modify molecule-substrate interactions that contribute to the experimentally determined desorption energies. In this work, we explore the suitability of an additive nearest neighbor model for determining general coverage dependent single molecule desorption energies in non-covalent self-assembled monolayers (SAMs). These coverage dependent values serve as essential input to any model attempting to reproduce coverage dependent desorption or for understanding the time dependent desorption from a partially covered surface. This method is tested using a case study of coronene adsorbed on Au(111) and HOPG substrates with periodic DFT calculations. Calculations show that coronene exhibits coverage and substrate dependence in molecular desorption. We found that intermolecular contact energies in the coronene monolayer are not strongly influenced by the HOPG substrate, while coronene desorption on Au(111) exhibits strong cooperativity where the additive model fails.

Determining the Ratio of Two Types of Prostate Specific Antigens with Biochips and Gold Nanoparticles for Accurate Prostate Cancer Diagnosis

Prostate-specific antigen (PSA) is a well-known biomarker for prostate-cancer diagnosis. However, the serum PSA measurement alone is insufficient for accurate diagnoses because the correlation with cancer is weak within the gray zone-the biomarker level range wherein a clear-cut diagnosis is impossible. As such, accurate prostate cancer diagnosis has been supplemented by measurements of the ratio of two types of PSA: free PSA (fPSA) and complexed PSA (cPSA; α-1-antichymotrypsin-bound PSA). Herein, we describe a new method for measuring the ratio of these two types of PSA by using gold nanoparticles (AuNPs) and biochips. Both types of PSA in a sample are captured by the antibody immobilized on a biochip based on self-assembled monolayers on gold. fPSA and cPSA on the biochip are then distinguished by AuNPs that present antibodies against fPSA and cPSA, respectively. The presence of PSAs in a sample is detected with laser desorption/ionization time-of-flight mass spectrometry by observing reporter molecules, called amplification tags (Am-tags), on the AuNPs. One of the reporter molecules is an Am-tag without isotope labeling, and the other is a deuterium-labeled Am-tag (dAm-tag). These tags amplify mass signals so as to enhance the sensitivity of the method. A comparison of the mass intensities between the Am-tag and dAm-tag signals allows the determination of the ratio between fPSA and cPSA. We validated the selective measurement of fPSA and cPSA at different ratios in 50, 75, and 100 pM of total PSA (fPSA + cPSA) solutions corresponding to the gray zone in prostate-cancer diagnosis (4 - 10 ng/mL). Finally, the two types of PSA were spiked in fetal bovine serum at various ratios, and our strategy greatly afforded their accurate ratios as spiked based on a constructed calibration curve. These results clearly indicate that the strategy is applicable to human serum as a diagnostic and prognostic assay for prostate cancer.

Combination of Mass Signal Amplification and Isotope-Labeled Alkanethiols for the Multiplexed Detection of miRNAs

We report a fast and sensitive method for the multiplexed detection of miRNAs by combining mass signal amplification and isotope-labeled signal reporter molecules. In our strategy, target miRNAs are captured specifically by immobilized DNAs on gold nanoparticles (AuNPs), which carry a large number of small molecules, called amplification tags (Am-tags), as the reporter for the detection of target miRNAs. For multiplexed detection, we designed and synthesized four Am-tags containing 0, 4, 8, 12 isotopes so that they had same molecular properties but different molecular weights. By observing the mass signals of the Am-tags on AuNPs decorated along with different probe DNAs, four types of miRNAs in a sample could be easily discriminated, and the relative amounts of these miRNAs could be quantified. The practicability of our strategy was further verified by measuring the expression levels of two miRNAs in HUVECs in response to different CuSO₄ concentrations.

Tracking Invisible Transformations of Physisorbed Monolayers: LDI-TOF and MALDI-TOF Mass Spectrometry as Complements to STM Imaging

Triphenyleneethynylene (TPEE) derivatives bearing one long aliphatic chain on each terminal aryl ring and two short aliphatic chains on the central aryl ring (core chains) self-assemble single component and 1-D patterned, two-component, crystalline monolayers at the solution-graphite interface. The monolayer morphology directs the core chains off the graphite, making them accessible for chemical reactions but invisible to imaging by scanning tunneling microscopy (STM). This precludes using STM to monitor transformations of the core chains, either by reaction or solution-monolayer exchange of TPEE molecules. Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) successfully identifies TPEE compounds within physisorbed monolayers. The LDI-TOF spectra of TPEE monolayer-graphite samples exhibit strong molecular ion peaks and minimal fragmentation or background. LDI-TOF and STM techniques are combined to evaluate monolayer composition and morphology, track solution-monolayer exchange, to identify reaction products and to measure kinetics of chemical reactions at the solution-monolayer interface. LDI-TOF MS provides rapid qualitative evaluation of monolayer composition across a graphite substrate. Challenges to quantitative composition evaluation by LDI-TOF include compound-specific light absorption, surface desorption/ionization and fragmentation characteristics. For some, but not all, compounds, applying matrix onto a self-assembled monolayer increases molecular ion intensities and affords more accurate assessment of monolayer composition via matrix assisted laser desorption/ionization (MALDI) MS. Matrix addition precludes subsequent chemical or STM studies of the monolayer, whereas reactions and STM may be performed at nonirradiated regions following LDI-TOF measurements. LDI- and MALDI-TOF MS are useful complements to STM and are easily implemented tools for study of physisorbed monolayers.

Plasmonic detection and visualization of directed adsorption of charged single nanoparticles to patterned surfaces

It has recently been shown that surface plasmon microscopy (SPM) allows single nanoparticles (NPs) on sensor surfaces to be detected and analyzed. The authors have applied this technique to study the adsorption of single metallic and plastic NPs. Binding of gold NPs (40, 60 and 100 nm in size) and of 100 nm polystyrene NPs to gold surfaces modified by differently ω-functionalized alkyl thiols was studied first. Self-assembled monolayers (SAM) with varying terminal functions including amino, carboxy, oligo(ethylene glycol), methyl, or trimethylammonium groups were deposited on gold films to form surfaces possessing different charge and hydrophobicity. The affinity of NPs to these surfaces depends strongly on the type of coating. SAMs terminated with trimethylammonium groups and carboxy group display highly different affinity and therefore were preferred when creating patterned charged surfaces. Citrate-stabilized gold NPs and sulfate-terminated polystyrene NPs were used as negatively charged NPs, while branched polyethylenimine-coated silver NPs were used as positively charged NPs. It is shown that the charged patterned areas on the gold films are capable of selectively adsorbing oppositely charged NPs that can be detected and analyzed with an ~1 ng⋅mL⁻¹ detection limit.

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On-chip enzymatic assay for chloramphenicol acetyltransferase using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Herein, we report a chloramphenicol (CAP) acetyltransferase (CAT) activity assay based on self-assembled monolayers on gold as an alternative to conventional CAT reporter gene assay systems, which sometimes require toxic materials and complicated steps that limit their use. A CAP derivative presented on a monolayer was converted to the acetylated CAP by CAT in the presence of acetyl-CoA. The conversion was directly monitored by observing the molecular weight changes in CAP using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. CAT activity was determined under various reaction conditions by changing reaction times, CAT and acetyl-CoA concentrations. As a practical application, we identified gene expression in bacteria that were transformed with pCAT plasmid DNA. Our strategy can provide a simple and rapid assay that eliminates some commonly used but potentially detrimental steps in enzymatic assays, such as radioactive labeling and complicated separation and purification of analytes prior to detection.