The power of fluorescence excitation–emission matrix (EEM) spectroscopy in the identification and characterization of complex mixtures of fluorescent silver clusters

Silver and gold clusters have received a lot of recent attention for their use in biomedical imaging. However, crude solutions of clusters are often complex mixtures, leading to discrepancies in their identification and characterization; important factors in determining their utility in biological applications. In the present study, silver clusters were separated for analysis using reverse-phase high performance liquid chromatography, which has previously been implemented in the efficient separation of gold clusters. Using fluorescence excitation–emission matrix (EEM) spectroscopy, we have demonstrated that a certain family of glutathione-protected silver clusters, previously thought to be one optically distinct species, is better described as a complex mixture of at least three distinct silver cluster species, each possessing unique optical properties. Based on these findings, EEM spectroscopy can be implemented as a powerful technique for determining the purity of complex mixtures, especially when other techniques, including mass spectrometry, fail to provide adequate characterization of a given material.

EEM spectroscopy can be implemented as a powerful technique for determining the purity of complex mixtures, especially when other techniques, including mass spectrometry, fail to provide adequate characterization of a given material.

3D-Printed Paper Spray Ionization Cartridge with Integrated Desolvation Feature and Ion Optics

In this work we present the application of 3D-printing for the miniaturization and functionalization of an ion source for (portable) mass spectrometry (MS). Two versions of a 3D-printed cartridge for paper spray ionization (PSI) are demonstrated, assessed, and compared. We first focus on the use of 3D-printing to enable the integration of an embedded electrostatic lens and a manifold for internal sheath gas distribution and delivery. Cartridges with and without a sheath gas manifold and an electrostatic lens are compared with respect to analytical performance and operational flexibility. The sensitivity and limit of detection are improved in the cartridge with an electrostatic lens and sheath gas manifold compared to the cartridge without (15% and over 6.5× smaller, respectively). The use of these focusing elements also improved the average spray stability. Furthermore, the range of potentials required for PSI was lower, and the distance to the MS orifice over which spray could be obtained was larger. Importantly, both setups allowed quantification of a model drug in the ng/mL range with single-stage MS, after correction for spray instability. Finally, we believe that this work is an example of the impact that 3D-printing will have on the future of analytical device fabrication, miniaturization, and functionalization.

Controlled, synchronized actuation of microdroplets by gravity in a superhydrophobic, 3D-printed device

Droplet manipulation over open surfaces allows one to perform assays with a large degree of control and high throughput, making them appealing for applications in drug screening or (bio)analysis. However, the design, manufacturing and operation of these systems comes with high technical requirements. In this study we employ a commercial, low-friction, superhydrophobic coating, Ultra-Ever Dry^®, on a 3D-printed microfluidic device. The device features individual droplet compartments, which allow the manipulation of discrete droplets (10-50 μL) actuated by gravity alone. Simply by angling the device to normal in a 3D-printed holder and rocking in a "to and fro"-fashion, a sequence of droplets can be individually transferred to an electrochemical microelectrode detector and then to waste, while preserving the (chronological) order of samples. Multiple biological fluids (i.e. human saliva, urine and rat blood and serum) were successfully tested for compatibility with the device and actuation mechanism, demonstrating low slip angles and high contact angles. Biological matrix (protein) carryover was probed and effectively mitigated by incorporating aqueous rinse droplets as part of the analysis sequence. As a proof-of-concept, the enzyme-coupled, amperometric detection of glucose was carried out on individual rat serum droplets, enabling total analysis in ≈30 min, including calibration. The device is readily customizable, and the integration of droplet generation techniques and other sensor systems for different analytes of interest or applications can be realized in a plug and play fashion.

Magnetically manipulated droplet splitting on a 3D-printed device to carry out a complexometric assay

A method for performing droplet actuation, splitting, and dispensing using only magnetic force and physical confinement is reported. The combination of low-friction superhydrophobic surfaces and droplets containing superparamagnetic particles is demonstrated to reliably dispense droplets with a precision (≤6%) similar to standard air-displacement pipettes. The 3D printed microfluidic chips incorporate individual wells, a weir structure and differential channel depths to facilitate droplet splitting in differing ratios. Both empirical observations and numerical simulations show that the splitting is a combination of wetting and pressure differences. The method enables a parent drop to be dispensed and split into droplets ranging in size from 5-20 μL using different well volumes. Once dispensed/split the droplets can be further actuated, merged and mixed. An EDTA-based complexometric colorimetric titration for water hardness is conducted on-chip. The degree of colour change is then determined utilizing a cell phone camera and image analysis and used to calculate water hardness; this measurement was found to agree with the traditional, larger scale method. The simple, robust dispensing method is adaptable to other digital microfluidic assays.

Fabrication of Patterned Superhydrophobic/Hydrophilic Substrates by Laser Micromachining for Small Volume Deposition and Droplet-Based Fluorescence

The deposition of nanoliter and subnanoliter volumes is important in chemical and biochemical droplet-based microfluidic systems. There are several techniques that have been established for the deposition/generation of small volumes including the use of surfaces with patterned differences in wettability. Many such methods require complex and time-consuming lithographic techniques. Here, we present a facile method for the fabrication of superhydrophobic surfaces with patterned hydrophilic regions by laser micromachining. A comprehensive study of fabrication parameters (laser machining speed, laser power, and patch size) on the material, patch wettability, and droplet volume is presented. Patch sizes as small as 100 μm diameter and as large as 1500 μm diameter were investigated, and volumes as low as 400 pL were observed. As an example application of such patterned materials and the deposition of small volumes, halide salts were preconcentrated on the hydrophilic patches, and their fluorescence quenching constants were rapidly calculated using a 3D-printed device coupled to a fluorescence spectrometer.

Characterization of plasma proteins adsorbed onto biomaterials. By MALDI-TOFMS

The analysis of plasma proteins adsorbed onto a polyurethane (PU) biomaterial was performed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). This article marks the first study on MALDI-TOFMS analysis of multiple proteins adsorbed from plasma, in vitro, onto the surface of a biomaterial to easily enable their characterization. Plasma standards from three different hosts were placed in contact with non-porous PU, a model biomaterial. Following the use of washing protocols developed in our laboratory, the biomaterial was analyzed, directly, with MALDI-TOFMS. Proteins with molecular weights (Mr) ranging from ca. 6.5 to 150 kDa were observed in the mass spectra and characterized upon comparison with proteins of known Mr. The proteins observed were tentatively identified as those known to adsorb onto PU, both in vitro and in vivo. In an attempt to model in vivo sorption, the PU biomaterial was exposed to freshly collected canine plasma, in vitro, for different lengths of time. Corresponding MALDI-TOFMS spectra displayed increasing protein signal for a number of different proteins with increasing times of exposure to plasma. This method provided qualitative and semi-quantitative analysis of the proteins adsorbed onto the biomaterial surface.

Article

An alternative sampling technique for fingerprint analysis of complex mixtures of petroleum-based products is presented. Fingerprint chromatograms of the volatile organic compounds (VOCs) for class 0-5 accelerants can be readily obtained using the recently developed inside needle capillary adsorption trap (INCAT) device. The concentration of VOCs from the static headspace of these complex mixtures, onto the adsorbing carbon coating of the INCAT device, avoids some problems associated with conventional headspace gas chromatography (HS-GC). The INCAT sampling method is a solventless extraction technique in which the analytes adsorbed inside the device are thermally desorbed inside the heated injection port of a gas chromatograph (GC), or a gas chromatograph coupled with a mass spectrometer (GC-MS). The presented method of active sampling with the INCAT device was used to obtain fingerprint chromatograms of the headspace of various gasolines, paint thinners, and lighter fluids. The static headspace chromatograms, from active sampling with the INCAT device, were compared with direct liquid injection of the samples. The INCAT chromatograms showed preferential adsorption of aromatic compounds over lighter, more volatile, but less polar compounds. Each class of the accelerant can be positively identified from the fingerprint chromatogram using only a few characteristic peaks. In addition, the ability to detect these volatile components of gasoline in aqueous samples at environmentally significant levels was also investigated.Key words: VOC, fingerprinting, INCAT, petroleum, headspace GC.

Characterization of hemoglobin variants by MALDI-TOF MS using a polyurethane membrane as the sample support

A new method for the sampling and off-site analysis of hemoglobin variants by mass spectrometry is reported. This technique uses a nonporous polyurethane membrane as the collection device and transportation medium of a blood sample for analysis. The same membrane is then used as the MALDI-TOF MS sample support for mass spectrometric analysis. Minimal invasive sample collection is afforded by collecting less than 1 microL of blood using a common lancet device. MALDI-TOF MS is performed directly on the membrane, after washing off the interfering plasma components, followed by the addition of matrix. This reduces the time of analysis and prevents sample loss. Enzymatic digestion can be performed directly on the membrane, using in this case trypsin, allowing for further characterization of the sample. The method is much less invasive compared to drawing blood with a syringe. The sample may be transported to the laboratory by regular mail, and thus the method can serve remote locations. We demonstrate the procedure by characterizing the Hb Shepherds Bush hemoglobin variant, b74-(E18)Gly-->Asp.

Use of a non-porous polyurethane membrane as a sample support for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of peptides and proteins

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) of proteins and peptides was performed on samples deposited onto non-porous ether-type polyurethane (PU) membranes. Spectra obtained using PU membranes showed that mass resolution and accuracy were equivalent to values observed using a metal target, and superior to those obtained using poly(vinylidene difluoride) (PVDF) membranes. A small apparent increase in the mass of proteins and also loss of resolution were observed at very high laser irradiance due to charging, but were not observed under normal conditions. Analysis of NaCl-doped standards demonstrated that PU membranes yielded better results than a metallic target for salt-containing solutions. Relatively strong hydrophobic interactions between the proteins and peptides and the PU membrane allowed the incorporation of a washing step. This step allowed for the removal of salts and buffer components and thus provided an increase in resolution and mass accuracy. Digestion of citrate synthase (a protein of molecular weight 47,886) with trypsin was performed directly on the surface of the membrane for variable periods of time, and characteristic peptide fragments were observed by MALDI-TOFMS. Delayed extraction was used to increase the resolution and to permit more accurate mass assignments for those fragments. The use of PU membranes for MALDI-TOFMS analysis of proteins with higher molecular weights is also demonstrated.