Combined atomic force microscope-based topographical imaging and nanometer-scale resolved proximal probe thermal desorption/electrospray ionization-mass spectrometry

Imaging with mass spectrometry: Which ionization technique is best?

The use of mass spectrometry (MS) to acquire molecular images of biological tissues and other substrates has developed into an indispensable analytical tool over the past 25 years. Imaging mass spectrometry technologies are widely used today to study the in situ spatial distributions for a variety of analytes. Early MS images were acquired using secondary ion mass spectrometry and matrix-assisted laser desorption/ionization. Researchers have also designed and developed other ionization techniques in recent years to probe surfaces and generate MS images, including desorption electrospray ionization (DESI), nanoDESI, laser ablation electrospray ionization, and infrared matrix-assisted laser desorption electrospray ionization. Investigators now have a plethora of ionization techniques to select from when performing imaging mass spectrometry experiments. This brief perspective will highlight the utility and relative figures of merit of these techniques within the context of their use in imaging mass spectrometry.

Influence of Ion Source Geometry on the Repeatability of Topographically Guided LAESI-MSI

Spatially resolving the relative distribution of analyte molecules in biological matter holds great promise in the life sciences. Mass spectrometry imaging (MSI) is a technique that can provide such spatial resolution but remains underused in fields such as chemical ecology, as traditional MSI sample preparation is often chemically or morphologically invasive. Laser ablation electrospray ionization (LAESI)-MSI is a variation of MSI particularly well-suited for situations where chemical sample preparation is too invasive but provides new challenges related to the repeatability of measurement outcomes. We assess the repeatability of LAESI-MSI by sampling a droplet of [ring-¹³C₆]l-phenylalanine with known concentration and expressing the resulting variability as a coefficient of variation, c_v. In doing so, we entirely eliminate variability caused by surface morphology or underlying true sample gradients. We determine the limit of detection (LOD) for¹³C₆-Phe by sampling from droplets with successively decreasing but known concentration. We assess the influence of source geometry on the LOD and repeatability by performing these experiments using four distinct variations of sources: one commercial and three custom-built ones. Finally, we extend our study to leaf and stem samples Arabidopsis thaliana and Gossypium hirsutum. We overcome the challenges of LAESI associated with three-dimensional surface morphology by relying on work previously published. Our measurements on both controlled standard and realistic samples give strong evidence that LAESI-MSI's repeatability in current implementations is insufficient for MSI in chemical ecology.

Automated 3D Sampling and Imaging of Uneven Sample Surfaces with LA-REIMS

The analysis of samples with large height variations remains a challenge for mass spectrometry imaging (MSI), despite many technological advantages. Ambient sampling and ionization MS techniques allow for the molecular analysis of sample surfaces with height variations, but most techniques lack MSI capabilities. We developed a 3D MS scanner for the automated sampling and imaging of a 3D surface with laser-assisted rapid evaporative ionization mass spectrometry (LA-REIMS). The sample is moved automatically with a constant distance between the laser probe and sample surface in the 3D MS Scanner. The topography of the surface was scanned with a laser point distance sensor to define the MS measurement points. MS acquisition was performed with LA-REIMS using a surgical CO₂ laser coupled to a qTOF instrument. The topographical scan and MS acquisition can be completed within 1 h using the 3D MS scanner for 300 measurement points on uneven samples with a spatial resolution of 2 mm in the top view, corresponding to 22.04 cm². Comparison between the automated acquisition with the 3D MS scanner and manual acquisition by hand showed that the automation resulted in increased reproducibility between the measurement points. 3D visualizations of molecular distributions related to structural differences were shown for an apple, a marrowbone, and a human femoral head to demonstrate the imaging feasibility of the system. The developed 3D MS scanner allows for the automated sampling of surfaces with uneven topographies with LA-REIMS, which can be used for the 3D visualization of molecular distributions of these surfaces.

Nanoscale Mass Spectrometry Multimodal Imaging via Tip-Enhanced Photothermal Desorption

Materials ranging from adhesives, pharmaceuticals, lubricants, and personal care products are traditionally studied using macroscopic characterization techniques. However, their functionality is in reality defined by details of chemical organization on often noncrystalline matter with characteristic length scales on the order of microns to nanometers. Additionally, these materials are traditionally difficult to analyze using standard vacuum-based approaches that provide nanoscale chemical characterization due to their volatile and beam-sensitive nature. Therefore, approaches that operate under ambient conditions need to be developed that allow probing of nanoscale chemical phenomena and correlated functionality. Here, we demonstrate a tool for probing and visualizing local chemical environments and correlating them to material structure and functionality using advanced multimodal chemical imaging on a combined atomic force microscopy (AFM) and mass spectrometry (MS) system using tip-enhanced photothermal desorption with atmospheric pressure chemical ionization (APCI). We demonstrate enhanced performance metrics of the technique for correlated imaging and point sampling and illustrate the applicability for the analysis of trace chemicals on a human hair, additives in adhesives on paper, and pharmaceuticals samples notoriously difficult to analyze in a vacuum environment. Overall, this approach of correlating local chemical environments to structure and functionality is key to advancing research in many fields ranging from biology, to medicine, to material science.

Nanomechanical sampling of material for nanoscale mass spectrometry chemical analysis

The ability to spatially resolve the chemical distribution of compounds on a surface is important in many applications ranging from biological to material science. To this extent, we have recently introduced a hybrid atomic force microscopy (AFM)-mass spectrometry (MS) system for direct thermal desorption and pyrolysis of material with nanoscale chemical resolution. However, spatially resolved direct surface heating using local thermal desorption becomes challenging on material surfaces with low melting points, because the material will undergo a melting phase transition due to heat dissipation prior to onset of thermal desorption. Therefore, we developed an approach using mechanical sampling and collection of surface materials on an AFM cantilever probe tip for real-time analysis directly from the AFM tip. This approach allows for material to be concentrated directly onto the probe for subsequent MS analysis. We evaluate the performance metrics of the technique and demonstrate localized MS sampling from a candelilla wax matrix containing UV stabilizers avobenzone and oxinoxate from areas down to 250 nm × 250 nm. Overall, this approach removes heat dissipation into the bulk material allowing for a faster desorption and concentration of the gas phase analyte from a single heating pulse enabling higher signal levels from a given amount of material in a single sampling spot.Graphical abstract.

Rapid quantification of acetaminophen in plasma using solid-phase microextraction coupled with thermal desorption electrospray ionization mass spectrometry

RATIONALE

Solid-phase microextraction coupled with thermal desorption electrospray ionization tandem mass spectrometry (SPME-TD-ESI-MS/MS) is proposed as a novel method for the rapid quantification of acetaminophen in plasma samples from a pharmacokinetics (PK) study.

METHODS

Traces of acetaminophen were concentrated on commercial fused-silica fibers coated with a polar polyacrylate (PA) polymer using direct immersion SPME. No agitation, heating, addition of salt, or adjustment of the pH of the sample solution was applied during the extraction. Any acetaminophen absorbed on the SPME fibers was subsequently desorbed and detected by TD-ESI-MS/MS.

RESULTS

Parameters of the absorption, sensitivity, reproducibility, and linearity for the SPME-TD-ESI-MS/MS method were evaluated. The time required to complete a TD-ESI-MS/MS analysis was less than 30 seconds. Matrix-matching calibration was performed to calculate the concentration of acetaminophen in the sample. A linear calibration curve with a concentration range of 100-10,000 ng/mL was constructed to calculate the quantity of acetaminophen. The SPME-TD-ESI-MS quantification results for acetaminophen in plasma were in good agreement with those obtained by the conventional LC/MS/MS method.

CONCLUSIONS

With the proposed method, a 10-min SPME time was enough to achieve the lower limit of quantitation (i.e. 100 ng/mL) and for a complete PK profiling of acetaminophen. A shorter extraction time could be achieved by applying agitation, heating, adding salt, or adjusting the pH of the sample solution to enhance analyte absorption efficiency. The time required to detect acetaminophen on the SPME fiber was less than 30 s, allowing the rapid quantification of acetaminophen in plasma with good accuracy.

Correlated Materials Characterization via Multimodal Chemical and Functional Imaging

Multimodal chemical imaging simultaneously offers high-resolution chemical and physical information with nanoscale and, in select cases, atomic resolution. By coupling modalities that collect physical and chemical information, we can address scientific problems in biological systems, battery and fuel cell research, catalysis, pharmaceuticals, photovoltaics, medicine, and many others. The combined systems enable the local correlation of material properties with chemical makeup, making fundamental questions of how chemistry and structure drive functionality approachable. In this Review, we present recent progress and offer a perspective for chemical imaging used to characterize a variety of samples by a number of platforms. Specifically, we present cases of infrared and Raman spectroscopies combined with scanning probe microscopy; optical microscopy and mass spectrometry; nonlinear optical microscopy; and, finally, ion, electron, and probe microscopies with mass spectrometry. We also discuss the challenges associated with the use of data originated by the combinatorial hardware, analysis, and machine learning as well as processing tools necessary for the interpretation of multidimensional data acquired from multimodal studies.

Local collection, reaction and analysis with theta pipette emitters

A mobile nanofluidic device based on theta pipettes was developed for "collect-react-analyze" measurements of small volumes of a sample collected locally from biological samples. Specifically, we demonstrate execution of local reactions inside single cells and on Pseudomonas aeruginosa biofilms for targeted analysis of metabolites. Nanoliter volumes of the sample, post-reaction, were delivered to a mass spectrometer via electrospray ionization (ESI) for chemical analysis. A new strategy was developed where the additional barrel of a theta pipette was utilized both to enable chemical manipulations after sample collection and to electrospray the nanoliter sample volumes collected directly from the pipette tip. This strategy proved a robust method for ESI from nanometer sized tips without clogging or degradation of the emitter and obviated the need to coat glass pipettes with a conductive metal coating. Chemical reactions investigated include acid catalyzed degradation of oligosaccharides inside the pipette tip to increase the detection sensitivity of minor metabolites found in Allium cepa cells. Additionally, phenylboronic acid complexation of carbohydrates from single cells and liposaccharides from biofilms was also performed inside the pipette tip for selective detection of carbohydrates and liposaccharides with cis-diols.

Probe-Substrate Distance Control in Desorption Electrospray Ionization

We introduce probe-substrate distance (D_ps)-control to desorption electrospray ionization (DESI) and report a systematic investigation of key experimental parameters. Examination of voltage, flow rate, and nebulizing gas pressure suggests as D_ps decreases, the distance-dependent spray current increases, until a critical point. At the critical point the relationship inverts, and the spray current decreases as the probe moves closer to the surface due to constriction of solution flow by the nebulizing gas. D_ps control was used to explore the use of spray current as a signal for feedback positioning, while mass spectrometry imaging was performed simultaneously. Further development of this technique is expected to find application in study of structure-function relationships for clinical diagnostics, biological investigation, and materials characterization. Graphical abstract ᅟ.

Tip-enhanced ablation and ionization mass spectrometry for nanoscale chemical analysis

Spectroscopic methods with nanoscale lateral resolution are becoming essential in the fields of physics, chemistry, geology, biology, and materials science. However, the lateral resolution of laser-based mass spectrometry imaging (MSI) techniques has so far been limited to the microscale. This report presents the development of tip-enhanced ablation and ionization time-of-flight mass spectrometry (TEAI-TOFMS), using a shell-isolated apertureless silver tip. The TEAI-TOFMS results indicate the capability and reproducibility of the system for generating nanosized craters and for acquiring the corresponding mass spectral signals. Multi-elemental analysis of nine inorganic salt residues and MSI of a potassium salt residue pattern at a 50-nm lateral resolution were achieved. These results demonstrate the opportunity for the distribution of chemical compositions at the nanoscale to be visualized.

Label-free molecular imaging of the kidney

In this review, we will highlight technologies that enable scientists to study the molecular characteristics of tissues and/or cells without the need for antibodies or other labeling techniques. Specifically, we will focus on matrix-assisted laser desorption/ionization imaging mass spectrometry, infrared spectroscopy, and Raman spectroscopy.

Visualizing molecular distributions for biomaterials applications with mass spectrometry imaging: a review

Mass spectrometry imaging (MSI) is a rapidly emerging field that is continually finding applications in new and exciting areas. The ability of MSI to measure the spatial distribution of molecules at or near the surface of complex substrates makes it an ideal candidate for many applications, including those in the sphere of materials chemistry. Continual development and optimization of both ionization sources and analyzer technologies have resulted in a wide array of MSI tools available, both commercially available and custom-built, with each configuration possessing inherent strengths and limitations. Despite the unique potential of MSI over other chemical imaging methods, their potential and application to (bio)materials science remains in our view a largely underexplored avenue. This review will discuss these techniques enabling high parallel molecular detection, focusing on those with reported uses in (bio)materials chemistry applications and highlighted with select applications. Different technologies are presented in three main sections; secondary ion mass spectrometry (SIMS) imaging, matrix-assisted laser desorption ionization (MALDI) MSI, and emerging MSI technologies with potential for biomaterial analysis. The first two sections (SIMS and MALDI) discuss well-established methods that are continually evolving both in technological advancements and in experimental versatility. In the third section, relatively new and versatile technologies capable of performing measurements under ambient conditions will be introduced, with reported applications in materials chemistry or potential applications discussed. The aim of this review is to provide a concise resource for those interested in utilizing MSI for applications such as biomimetic materials, biological/synthetic material interfaces, polymer formulation and bulk property characterization, as well as the spatial and chemical distributions of nanoparticles, or any other molecular imaging application requiring broad chemical speciation.

Towards Practical Endoscopic Mass Spectrometry

In this paper, we briefly review the remote mass spectrometric techniques that are viable to perform "endoscopic mass spectrometry," i.e., in-situ and in-vivo MS analysis inside the cavity of human or animal body. We also report our experience with a moving string sampling probe for the remote sample collection and the transportation of adhered sample to an ion source near the mass spectrometer. With a miniaturization of the probe, the method described here has the potential to be fit directly into a medical endoscope.

Improved spatial resolution for spot sampling in thermal desorption atomic force microscopy - mass spectrometry via rapid heating functions

The key to advancing materials is to understand and control their structure and chemistry. However, thorough chemical characterization is challenging since existing techniques characterize only a few properties of the specimen, thereby necessitating multiple measurement platforms to acquire the necessary information. The multimodal combination of atomic force microscopy (AFM) and mass spectrometry (MS) transcends existing analytical capabilities for nanometer scale spatially resolved correlation of the chemical and physical properties of a sample surface. One such hybrid system employs heated AFM cantilevers for thermal desorption (TD) sampling of molecules from a surface and subsequent gas phase ionization and detection of the liberated species by MS. Herein, we report on the use of voltage pulse trains to tailor cantilever heating such that spot sampling size was reduced and mass spectral signal was improved compared to constant voltage, static heating of the cantilever. Desorption efficiency (DE), defined as the quotient of the mass spectral signal intensity and the volume of the desorption crater, was used to judge the effectiveness of a particular tailored heating function. To guide the development and optimization of the heating functions and aid in interpreting experimental results, a 1D finite element model was developed that predicted the cantilever response to different heating functions. Three tailored heating functions that used different combinations, magnitudes, and durations of rectangular voltage pulses, were used for surface spot sampling. The resultant sampling spot size and DE were compared to the same metrics obtained with the conventional method that uses a single voltage pulse. Using a model system composed of a thin film of ink containing pigment yellow 74 as a model system, desorption craters shrunk from 2 μm, using the conventional approach, to 310 nm using the optimum tailored heating function. This same pulsed heating function produced a 381× improvement in the DE and an 8× improvement in spatial resolution compared to the conventional heating approach showing that signal/amount of material sampled was improved significantly by this new cantilever heating strategy.

High sensitivity field asymmetric ion mobility spectrometer

A high sensitivity field asymmetric ion mobility spectrometer (FAIMS) was designed, fabricated, and tested. The main components of the system are a 10.6 eV UV photoionization source, an ion filter driven by a high voltage/high frequency n-MOS inverter circuit, and a low noise ion detector. The ion filter electronics are capable to generate square waveforms with peak-to-peak voltages up to 1000 V at frequencies up to 1 MHz with adjustable duty cycles. The ion detector current amplifier has a gain up to 10¹² V/A with an effective equivalent input noise level down to about 1 fA/Hz^1/2 during operation with the ion filter at the maximum voltage and frequency. The FAIMS system was characterized by detecting different standard chemical compounds. Additionally, we investigated the use of a synchronous modulation/demodulation technique to improve the signal-to-noise ratio in FAIMS measurements. In particular, we implemented the modulation of the compensation voltage with the synchronous demodulation of the ion current. The analysis of the measurements at low concentration levels led to an extrapolated limit of detection for acetone of 10 ppt with an averaging time of 1 s.

In vivo endoscopic mass spectrometry using a moving string sampling probe

A novel moving string sampling probe and sample transportation system for performing in situ and in vivo endoscopic MS.

Constant-Distance Mode Nanospray Desorption Electrospray Ionization Mass Spectrometry Imaging of Biological Samples with Complex Topography

A new approach for constant-distance mode mass spectrometry imaging (MSI) of biological samples using nanospray desorption electrospray ionization (nano-DESI) was developed by integrating a shear-force probe with the nano-DESI probe. The technical concept and basic instrumental setup, as well as the general operation of the system are described. Mechanical dampening of resonant oscillations due to the presence of shear forces between the probe and the sample surface enabled the constant-distance imaging mode via a computer-controlled closed-feedback loop. The capability of simultaneous chemical and topographic imaging of complex biological samples is demonstrated using living Bacillus subtilis ATCC 49760 colonies on agar plates. The constant-distance mode nano-DESI MSI enabled imaging of many metabolites, including nonribosomal peptides (surfactin, plipastatin, and iturin) on the surface of living bacterial colonies, ranging in diameter from 10 to 13 mm, with height variations up to 0.8 mm above the agar plate. Co-registration of ion images to topographic images provided higher-contrast images. Based on this effort, constant-mode nano-DESI MSI proved to be ideally suited for imaging biological samples of complex topography in their native states.

Co-registered Topographical, Band Excitation Nanomechanical, and Mass Spectral Imaging Using a Combined Atomic Force Microscopy/Mass Spectrometry Platform

The advancement of a hybrid atomic force microscopy/mass spectrometry imaging platform demonstrating the co-registered topographical, band excitation nanomechanical, and mass spectral imaging of a surface using a single instrument is reported. The mass spectrometry-based chemical imaging component of the system utilized nanothermal analysis probes for pyrolytic surface sampling followed by atmospheric pressure chemical ionization of the gas-phase species produced with subsequent mass analysis. The basic instrumental setup and operation are discussed, and the multimodal imaging capability and utility are demonstrated using a phase-separated polystyrene/poly(2-vinylpyridine) polymer blend thin film. The topography and band excitation images showed that the valley and plateau regions of the thin film surface were comprised primarily of one of the two polymers in the blend with the mass spectral chemical image used to definitively identify the polymers at the different locations. Data point pixel size for the topography (390 nm × 390 nm), band excitation (781 nm × 781 nm), and mass spectrometry (690 nm × 500 nm) images was comparable and submicrometer in all three cases, but the data voxel size for each of the three images was dramatically different. The topography image was uniquely a surface measurement, whereas the band excitation image included information from an estimated 20 nm deep into the sample and the mass spectral image from 110 to 140 nm in depth. Because of this dramatic sampling depth variance, some differences in the band excitation and mass spectrometry chemical images were observed and were interpreted to indicate the presence of a buried interface in the sample. The spatial resolution of the chemical image was estimated to be between 1.5 and 2.6 μm, based on the ability to distinguish surface features in that image that were also observed in the other images.

Nanopipettes: probes for local sample analysis

Nanopipettes are demonstrated as probes for local mass spectrometric analysis with potential for small-scale extraction of analytes from single cells, tissue and organisms.

Nanopipettes (pipettes with diameters <1 μm) were explored as pressure-driven fluid manipulation tools for sampling nanoliter volumes of fluids. The fundamental behavior of fluids confined in the narrow channels of the nanopipette shank was studied to optimize sampling volume and probe geometry. This method was utilized to collect nanoliter volumes (<10 nL) of sample from single Allium cepa cells and live Drosophila melanogaster first instar larvae. Matrix assisted laser desorption/ionization-mass spectrometry (MALDI-MS) was utilized to characterize the collected sample. The use of nanopipettes for surface sampling of mouse brain tissue sections was also explored. Lipid analyses were performed on mouse brain tissues with spatial resolution of sampling as small as 50 μm. Nanopipettes were shown to be a versatile tool that will find further application in studies of sample heterogeneity and population analysis for a wide range of samples.

Atomic force microscope controlled topographical imaging and proximal probe thermal desorption/ionization mass spectrometry imaging

This paper reports on the development of a hybrid atmospheric pressure atomic force microscopy/mass spectrometry imaging system utilizing nanothermal analysis probes for thermal desorption surface sampling with subsequent atmospheric pressure chemical ionization and mass analysis. The basic instrumental setup and the general operation of the system were discussed, and optimized performance metrics were presented. The ability to correlate topographic images of a surface with atomic force microscopy and a mass spectral chemical image of the same surface, utilizing the same probe without moving the sample from the system, was demonstrated. Co-registered mass spectral chemical images and atomic force microscopy topographical images were obtained from inked patterns on paper as well as from a living bacterial colony on an agar gel. Spatial resolution of the topography images based on pixel size (0.2 μm × 0.8 μm) was better than the resolution of the mass spectral images (2.5 μm × 2.0 μm), which were limited by current mass spectral data acquisition rate and system detection levels.

Microscopy ambient ionization top-down mass spectrometry reveals developmental patterning

There is immense cellular and molecular heterogeneity in biological systems. Here, we demonstrate the utility of integrating an inverted light microscope with an ambient ionization source, nanospray electrospray desorption ionization, attached to a high-resolution mass spectrometer to characterize the molecular composition of mouse spinal cords. We detected a broad range of molecules, including peptides and proteins, as well as metabolites such as lipids, sugars, and other small molecules, including S-adenosyl methionine and glutathione, through top-down MS. Top-down analysis revealed variation in the expression of Hb, including the transition from fetal to adult Hb and heterogeneity in Hb subunits consistent with the genetic diversity of the mouse models. Similarly, temporal changes to actin-sequestering proteins β-thymosins during development were observed. These results demonstrate that interfacing microscopy with ambient ionization provides the means to perform targeted in situ ambient top-down mass spectral analysis to study the pattern of proteins, lipids, and sugars in biologically heterogeneous samples.

Laser microdissection and atmospheric pressure chemical ionization mass spectrometry coupled for multimodal imaging

RATIONALE

Improvement in spatial resolution of atmospheric pressure molecular chemical imaging is required to resolve distinct surface features in the low micrometer and sub-micrometer scale. Laser capture microdissection systems have the capability to focus laser light to a few micrometers. This type of system, when employed for laser ablation (LA) mass spectrometry (MS)-based chemical imaging, has the potential to achieve high spatial resolution with multimodal optical and chemical imaging capability.

METHODS

A commercially available laser capture microdissection system was coupled to a modified ion source of a mass spectrometer. This design allowed for sampling of laser-ablated material via a transfer tube directly into the ionization region. Ionization of the ablated material was accomplished using atmospheric pressure chemical ionization (APCI).

RESULTS

Rhodamine 6G dye of red permanent marker ink in a laser etched pattern as well as cholesterol and phosphatidylcholine in a cerebellum mouse brain thin tissue section were identified and imaged from the mass spectral data. Employing a spot diameter of 8 µm using the 10× microscope cutting objective and lateral oversampling resulted in a pixel size of about 3.7 µm in the same dimension. Distinguishing between features approximately 13 µm apart in a cerebellum mouse brain thin tissue section was demonstrated in a multimodal fashion co-registering optical and mass spectral images.

CONCLUSIONS

A LA/APCI-MS system was developed that comprised a commercially available laser microdissection instrument for transmission geometry LA and a modestly modified ion source for secondary ionization of the ablated material. The set-up was successfully applied for multimodal imaging using the ability to co-register bright field, fluorescence and mass spectral chemical images on one platform.

Temperature measurements of heated microcantilevers using scanning thermoreflectance microscopy

We report the development of scanning thermoreflectance thermometry and its application for steady and dynamic temperature measurement of a heated microcantilever. The local thermoreflectance signal of the heated microcantilever was calibrated to temperature while the cantilever was under steady and periodic heating operation. The temperature resolution of our approach is 0.6 K, and the spatial resolution is 2 μm, which are comparable to micro-Raman thermometry. However, the temporal resolution of our approach is about 10 μsec, which is significantly faster than micro-Raman thermometry. When the heated microcantilever is periodically heated with frequency up to 100 kHz, we can measure both the in-phase and out-of-phase components of the temperature oscillation. For increasing heating frequency, the measured cantilever AC temperature distribution tends to be confined in the vicinity of the heater region and becomes increasingly out of phase with the driving signal. These results compare well with finite element simulations.

RGD-peptide functionalized graphene biomimetic live-cell sensor for real-time detection of nitric oxide molecules

It is always challenging to construct a smart functional nanostructure with specific physicochemical properties to real time detect biointeresting molecules released from live-cells. We report here a new approach to build a free-standing biomimetic sensor by covalently bonding RGD-peptide on the surface of pyrenebutyric acid functionalized graphene film. The resulted graphene biofilm sensor comprises a well-packed layered nanostructure, in which the RGD-peptide component provides desired biomimetic properties for superior human cell attachment and growth on the film surface to allow real-time detection of nitric oxide, an important signal yet short-life molecule released from the attached human endothelial cells under drug stimulations. The film sensor exhibits good flexibility and stability by retaining its original response after 45 bending/relaxing cycles and high reproducibility from its almost unchanged current responses after 15 repeated measurements, while possessing high sensitivity, good selectivity against interferences often existing in biological systems, and demonstrating real time quantitative detection capability toward nitric oxide molecule released from living cells. This study not only demonstrates a facial approach to fabricate a smart nanostructured graphene-based functional biofilm, but also provides a powerful and reliable platform to the real-time study of biointeresting molecules released from living cells, thus rendering potential broad applications in neuroscience, screening drug therapy effect, and live-cell assays.

Active capillary plasma source for ambient mass spectrometry

RATIONALE

Imaging mass spectrometry with high spatial resolution has become a rapidly developing area of mass spectrometric research. Many scientific and industrial problems deal with mass spectrometric analysis at ambient pressures. This requires efficient transport and ionization of small amounts of substance.

METHODS

An active sampling capillary based on a dielectric barrier discharge was constructed for ambient mass spectrometry. The capillary serves as an ionization source as well as an atmospheric interface of the mass spectrometer. The analyzed samples are transported through the sampling capillary due to the pressure difference between the atmospheric environment and the vacuum in the mass spectrometer. Ionization of the transported samples is provided by a low-temperature dielectric barrier discharge. This active capillary was shown to work in a robust fashion for ionizing both gas-phase and laser-ablated solid-phase samples.

RESULTS

The geometry of the electrodes was found to play a crucial role in ionization efficiency. The capillary was optimized in order to perform surface analysis of solid samples. Sensitivity tests were carried out to characterize different active capillary constructions. The sensitivity of the constructed active capillary was 0.1 ppb (relative concentration of a gaseous sample in ambient air). The active capillary was able to detect samples evaporated from a solid surface. With the active capillary source, it was possible to detect anthracene traces evaporated from a surface located 3 cm from the capillary inlet.

CONCLUSIONS

A plasma-based active capillary ionization source was constructed. This concept of an active sample inlet should broaden the range of applications of ambient mass spectrometry.

Thermoelectric voltage at a nanometer-scale heated tip point contact

We report thermoelectric voltage measurements between the platinum-coated tip of a heated atomic force microscope (AFM) cantilever and a gold-coated substrate. The cantilevers have an integrated heater-thermometer element made from doped single crystal silicon, and a platinum tip. The voltage can be measured at the tip, independent from the cantilever heating. We used the thermocouple junction between the platinum tip and the gold substrate to measure thermoelectric voltage during heating. Experiments used either sample-side or tip-side heating, over the temperature range 25-275 °C. The tip-substrate contact is ∼4 nm in diameter and its average measured Seebeck coefficient is 3.4 μV K(-1). The thermoelectric voltage is used to determine tip-substrate interface temperature when the substrate is either glass or quartz. When the non-dimensional cantilever heater temperature is 1, the tip-substrate interface temperature is 0.593 on glass and 0.125 on quartz. Thermal contact resistance between the tip and the substrate heavily influences the tip-substrate interface temperature. Measurements agree well with modeling when the tip-substrate interface contact resistance is 10(8) K W(-1).