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Spectral modeling of photoelectrons can serve as a valuable tool when combined with X-ray photoelectron spectroscopy (XPS) analysis. Herein, a new version of the NIST Simulation of Electron Spectra for Surface Analysis (SESSA 2.0) software, capable of directly simulating spherical multilayer NPs, was applied to model citrate stabilized Au/Ag-core/shell nanoparticles (NPs). The NPs were characterized using XPS and scanning transmission electron microscopy (STEM) to determine the composition and morphology of the NPs. The Au/Ag-core/shell NPs were observed to be polydispersed in size, nonspherical, and contain off-centered Au-cores. Using the average NP dimensions determined from STEM analysis, SESSA spectral modeling indicated that washed Au/Ag-core-shell NPs were stabilized with a 0.8 nm layer of sodium citrate and a 0.05 nm (one wash) or 0.025 nm (two wash) layer of adventitious hydrocarbon, but did not fully account for the observed XPS signal from the Au-core. This was addressed by a series of simulations and normalizations to account for contributions of NP nonsphericity and off-centered Au-cores. Both of these nonuniformities reduce the effective Ag-shell thickness, which effect the Au-core photoelectron intensity. The off-centered cores had the greatest impact for the particles in this study. When the contributions from the geometrical nonuniformities are included in the simulations, the SESSA generated elemental compositions that matched the XPS elemental compositions. This work demonstrates how spectral modeling software such as SESSA, when combined with experimental XPS and STEM measurements, advances the ability to quantitatively assess overlayer thicknesses for multilayer core-shell NPs and deal with complex, nonideal geometrical properties.

P53 phosphorylation plays an important role in many biological processes and might be used as a potential biomarker in clinical diagnoses. We report a new electrochemical immunosensor for ultrasensitive detection of phosphorylated p53 at Ser392 (phospho-p53(392)) based on graphene oxide (GO) as a nanocarrier in a multienzyme amplification strategy. Greatly enhanced sensitivity was achieved by using the bioconjugates featuring horseradish peroxidase (HRP) and p53(392) signal antibody (p53(392)Ab(2)) linked to functionalized GO (HRP-p53(392)Ab(2)-GO) at a high ratio of HRP/p53(392)Ab(2). After a sandwich immunoreaction, the HRP-p53(392)Ab(2)-GO captured onto the electrode surface produced an amplified electrocatalytic response by the reduction of enzymatically oxidized thionine in the presence of hydrogen peroxide. The increase of response current was proportional to the phospho-p53(392) concentration in the range of 0.02-2 nM with the detection limit of 0.01 nM, which was 10-fold lower than that of the traditional sandwich electrochemical measurement for p53(392). The amplified immunoassay developed in this work shows acceptable stability and reproducibility, and the assay results for phospho-p53(392) spiked in human plasma also show good recovery (92-103.8%). This simple and low-cost immunosensor shows great promise for detection of other phosphorylated proteins and clinical applications.

A novel electrochemical immunosensor for sensitive detection of cancer biomarker alpha-fetoprotein (AFP) is described that uses a graphene sheet sensor platform and functionalized carbon nanospheres (CNSs) labeled with horseradish peroxidase-secondary antibodies (HRP-Ab2). Greatly enhanced sensitivity for the cancer biomarker is based on a dual signal amplification strategy: first, the synthesized CNSs yielded a homogeneous and narrow size distribution, which allowed several binding events of HRP-Ab2 on each nanosphere. Enhanced sensitivity was achieved by introducing the multibioconjugates of HRP-Ab2-CNSs onto the electrode surface through "sandwich" immunoreactions. Second, functionalized graphene sheets used for the biosensor platform increased the surface area to capture a large amount of primary antibodies (Ab1), thus amplifying the detection response. On the basis of the dual signal amplification strategy of graphene sheets and the multienzyme labeling, the developed immunosensor showed a 7-fold increase in detection signal compared to the immunosensor without graphene modification and CNSs labeling. The proposed method could respond to 0.02 ng mL(-1) AFP with a linear calibration range from 0.05 to 6 ng mL(-1). This amplification strategy is a promising platform for clinical screening of cancer biomarkers and point-of-care diagnostics.

The ability to regulate how molecular chirality of enantiomeric amino acids operates in biological systems constitutes the basis of drug design for specific targeting. We report herein a nanoparticle-based strategy to regulate interparticle chiral recognition of enantiomers using enantiomeric cysteines (l and d) and gold nanoparticles as a model system. A key element of this strategy is the creation of a nanoscale environment either favoring or not favoring the preferential configuration of the pairwise zwitterionic dimerization of the enantiomeric cysteines adsorbed on gold nanoparticles as a footprint for interparticle chiral recognition. This recognition leads to interparticle assembly of the nanoparticles which is determined by the change in the nanoparticle surface plasmonic resonance. While the surface density and functionality of cysteines on gold nanoparticles are independent of chirality, the interparticle chiral recognition is evidenced by the sharp contrast between the interparticle homochiral and heterochiral assembly rates based on a first-order kinetic model. The structural properties for the homochiral and heterochiral assemblies of nanoparticles depend on the particle size, the cysteine chirality, and other interparticle binding conditions. The structural and thermodynamic differences between the homochiral and heterochiral interactions for the interparticle assemblies of nanoparticles were not only substantiated by spectroscopic characterizations of the adsorbed cysteine species but also supported by structures and enthalpies obtained from preliminary density functional theory calculations. The experimental-theoretical correlation between the interparticle reactivity and the enantiomeric ratio reveals that the chiral recognition is tunable by the nanoscale environment, which is a key feature of the nanoparticle-regulation strategy for the interparticle chiral recognition.

A novel electrochemical immunosensor for the detection of tumor necrosis factor-alpha (TNF-alpha) based on poly(guanine)-functionalized silica nanoparticles (NPs) label is presented. The detection of mouse TNF-alpha via immunological reaction is based on a dual signal amplification: (1) a large amount of guanine residues introduced on the electrode surface through sandwich immunoreaction and poly(guanine)-functionalized silica NP label; (2) Ru(bpy)3(2+)-induced catalytic oxidation of guanine, which results in great enhancement of anodic current. The synthesized silica NP conjugates were characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. These experiments confirmed that poly(guanine) and avidin were immobilized on the surface of silica NPs. The performance of the electrochemical immunosensor was evaluated and some experiment parameters (e.g., concentration of Ru(bpy)3(2+), incubation time of TNF-alpha, etc.) were optimized. The detection limit for TNF-alpha is found to be 5.0 x 10(-11) g mL(-1) (2.0 pM), which corresponds to 60 amol of TNF-alpha in 30 microL of sample. This immunosensor based on the poly(guanine)-functionalized silica NP label offers great promise for rapid, simple, cost-effective analysis of biological samples.