Electronic Structures of Cr(III) and V(II) Polypyridyl Systems: Undertones in an Isoelectronic Analogy.

A recently reported description of the photophysical properties of V2+ polypyridyl systems has highlighted several distinctions between isoelectronic, d3, Cr3+, and V2+ tris-homoleptic polypyridyl complexes of 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen). Here, we combine theory and experimental data to elucidate the differences in electronic structures. We provide the first crystallographic structures of the V2+ complexes [V(bpy)3](BPh4)2 (V-1B) and [V(phen)3](OTf)2 (V2) and observe pronounced trigonal distortion relative to analogous Cr3+ complexes. We use electronic absorption spectroscopy in tandem with TD-DFT computations to assign metal-ligand charge transfer (MLCT) properties of V-1B and V2 that are unique from the intraligand transitions, 4(3IL), solely observed in Cr3+ analogues. Our newly developed natural transition spin density (NTρα,ß) plots characterize both the Cr3+ and V2+ absorbance properties. A multideterminant approach to DFT assigns the energy of the 2E state of V-1B as stabilized through electron delocalization. We find that the profound differences in excited state lifetimes for Cr3+ and V2+ polypyridyls arise from differences in the characters of their lowest doublet states and pathways for intersystem crossing, both of which stem from trigonal structural distortion and metal-ligand π-covalency.

Hybrid paper and 3D-printed microfluidic device for electrochemical detection of Ag nanoparticle labels.

In the present article we report a new hybrid microfluidic device (hyFlow) comprising a disposable paper electrode and a three-dimensional (3D) printed plastic chip for the electrochemical detection of a magnetic bead-silver nanoparticle (MB-AgNP) bioconjugate. This hybrid device evolved due to the difficulty of incorporating micron-scale MBs into paper-only fluidic devices. Specifically, paper fluidic devices can entrap MB-containing conjugates within their cellulose or nitrocellulose fiber matrix. The hyFlow system was designed to minimize such issues and transport MB conjugates more efficiently to the electrochemical detection zone of the device. The hyFlow system retains the benefit of fluid transport by pressure-driven flow, however, no pump is required for its operation. The hyFlow device is capable of detecting either pre-formed MB-AgNP conjugates or conjugates formed in situ. The detection limit of AgNPs using this device is 12 pM, which represents just 22 AgNPs per MB.

Read-by-eye quantification of aluminum (III) in distance-based microfluidic paper-based analytical devices.

Herein we report the first two distance-based microfluidic paper-based analytical devices (µPADs) using fluorescence to quantify aluminum. In addition to their read-by-eye quantification, the devices are simple to fabricate, require no sample pretreatment or preconcentration, and have a shelf life of >5 months. The first device is designed in a "chemometer" format where the length of a fluorescent band linearly responds to an Al(III) concentration. The second device uses a radial design where the fluorescent diameter also linearly responds to an Al(III) concentration. The chemometer device has a detection limit of 2.5 ppm (100 µM) and a linear range from 2 to 54 ppm Al(III) (100 µM-1 mM), R2 = 0.989). The radial device has a detection limit of 0.9 ppm (33 µM) and a linear range from 2 to 24 ppm Al(III) (100-900 µM, R2 = 0.968). The utility of the µPADs were successfully demonstrated by measuring Al(III) in two water effluent samples from the Gold King Mine near Silverton, CO.

Multilayered Microfluidic Paper-Based Devices: Characterization, Modeling, and Perspectives.

Microfluidic paper-based analytical devices (µPADs) are simple but powerful analytical tools that are gaining significant recent attention due to their many advantages over more traditional monitoring tools. These include being inexpensive, portable, pump-free, and having the ability to store reagents. One major limitation of these devices is slow flow rates, which are controlled by capillary action in the hydrophilic pores of cellulosic paper. Recent investigations have advanced the flow rates in µPADs through the generation of a gap or channel between two closely spaced paper sheets. This multilayered format has opened up µPADs to new applications and detection schemes, where large gap sizes (>300 µm) provide at least 169× faster flow rates than single-layer µPADs, but do not conform to established mathematical models for fluid transport in porous materials, such as the classic Lucas-Washburn equation. In the present study, experimental investigations and analytical modeling are applied to elucidate the driving forces behind the rapid flow rates in these devices. We investigate a range of hypotheses for the systems fluid dynamics and establish a theoretical model to predict the flow rate in multilayered µPADs that takes into account viscous dissipation within the paper. Device orientation, sample addition method, and the gap height are found to be critical concerns when modeling the imbibition in multilayered devices.

Development of Paper-Based Analytical Devices for Minimizing the Viscosity Effect in Human Saliva.

Rationale: Saliva as a sample matrix is rapidly gaining interest for disease diagnosis and point-of-care assays because it is easy to collect (non-invasive) and contains many health-related biomarkers. However, saliva poses particular problems relative to more common urine and blood matrices, which includes low analyte concentrations, lack of understanding of biomolecule transportation and inherent viscosity variability in human samples. While several studies have sought to improve assay sensitivity, few have addressed sample viscosity specifically. The goal of this study is to minimize the effect of sample viscosity on paper-based analytical devices (PADs) for the measurement of pH and nitrite in human saliva. Methods: PADs were used to measure salivary pH from 5.0 to 10.0 with a universal indicator consisting of chlorophenol red, phenol red and phenolphthalein. Nitrite determination was performed using the Griess reaction. Artificial saliva with viscosity values between 1.54 and 5.10 mPa∙s was tested on the proposed PAD. To ensure the proposed PADs can be tailored for use in-field analysis, the devices were shipped to Australia and tested with human specimens. Results: Initial experiments showed that viscosity had a significant impact on the calibration curve for nitrite; however, a more consistent curve could be generated when buffer was added after the sample, irrespective of sample viscosity. The linear range for nitrite detection was 0.1 to 2.4 mg/dL using the improved method. The nitrite measurement in artificial saliva also showed a good correlation with the standard spectrophotometry method (p=0.8484, paired sample t-test, n=20). Measured pH values from samples with varying viscosities correlated well with the results from our pH meter. Conclusions: The inherent variation of salivary viscosity that impacts nitrite and pH results can be addressed using a simple washing step on the PAD without the need for complex procedures.

Design considerations for reducing sample loss in microfluidic paper-based analytical devices.

The field of microfluidic paper-based analytical devices (µPADs) is most notably characterized by portable and low-cost analysis; however, struggles to achieve the high sensitivity and low detection limits needs required for many environmental applications hinder widespread adoption of this technology. Loss of analyte to the device material represents an important problem impacting sensitivity. Critically, we found that at least 50% of a Ni(II) sample is lost when being transported down a 30 mm paper channel that is representative of structures commonly found in µPADs. In this work, we report simple strategies such as adding a waste zone, enlarging the detection zone, and using an elution step to increase device performance. A µPAD combining the best performing functionalities led to a 78% increase in maximum signal and a 28% increase in sensitivity when transporting Ni(II) samples. Using the optimized µPAD also led to a 94% increase in maximum signal for Mn(II) samples showing these modifications can be applied more generally.

Rapid flow in multilayer microfluidic paper-based analytical devices.

Microfluidic paper-based analytical devices (µPADs) are a versatile and inexpensive point-of-care (POC) technology, but their widespread adoption has been limited by slow flow rates and the inability to carry out complex in field analytical measurements. In the present work, we investigate multilayer µPADs as a means to generate enhanced flow rates within self-pumping paper devices. Through optical and electrochemical measurements, the fluid dynamics are investigated and compared to established flow theories within µPADs. We demonstrate a ∼145-fold increase in flow rate (velocity = 1.56 cm s-1, volumetric flow rate = 1.65 mL min-1, over 5.5 cm) through precise control of the channel height in a 2 layer paper device, as compared to archetypical 1 layer µPAD designs. These design considerations are then applied to a self-pumping sequential injection device format, known as a three-dimensional paper network (3DPN). These 3DPN devices are characterized through flow injection analysis of a ferrocene complex and anodic stripping detection of cadmium, exhibiting a 5× enhancement in signal compared to stationary measurements.

A selective distance-based paper analytical device for copper(II) determination using a porphyrin derivative.

Meso-tetrakis(1,2-dimethylpyrazolium-4-yl)porphyrin sulfonate (TDMPzP), a water-soluble porphyrin derivative, was synthesized and used as a colorimetric reagent for Cu2+ detection on a microfluidic paper-based analytical device (µPAD) using distance-based quantification. TDMPzP showed a high selectivity for Cu2+ detection in aqueous solutions. When Cu2+ was added to the TDMPzP under acidic conditions, a color change from green to a pink was observed by the naked eye. Under optimized conditions, the application of this system to a distance-based µPAD exhibited good analytical response. The presence of common metal ions (Al3+, Fe3+, Mg2+, Co2+, Mn2+, Zn2+, Pb2+, Cd2+, Sn2+, and Ni2+) did not interfere with Cu2+ detection within reasonable tolerance ratios. The lowest concentration of copper that could be measured was 1mgL-1 (1ppm) which meets the requirements for drinking water contamination regulations from the US Environmental Protection Agency (EPA) and World Health Organization (WHO) guidelines for drinking water. Real drinking water samples were analyzed to confirm the practical application of this system and the results showed good agreement with ICP-MS data. This distance-based µPAD based on TDMPzP for Cu2+ detection is convenient and effective for real-time drinking water analysis.

Critical interval of somal calcium transient after neurite transection determines B 104 cell survival.

Nerve cells may survive or die after axonal or dendritic transection. After neurite transection near (<50 mum) the cell body of Fura-2-loaded B 104 neuroblastoma (rat brain-derived) cells, the somal calcium concentration (SCC) undergoes a three-phase transient change: a rapid (0-0.15-min post-transection [PT]) rise phase, followed by an early (0.15--1.5-min PT) rapid decay phase, and succeeded by a late (1.5-60-min PT) slower decay phase that restores SCC to preinjury levels. The SCC in a critical interval (1.5-12.5 min PT) of the third transient phase correlates with cell fate, i.e., most transected cells that exclude dye (restore a barrier) and die have a significantly higher (P<0.005) SCC in this critical interval than do transected cells that exclude dye and survive at 24-hr PT. Loading BAPTA (chelation of somal Ca(2+)) before, but not after, the critical interval increases the percentage of cells that survive compared to that of cells transected without BAPTA loading. Furthermore, most transected cells that die despite successful barrier restoration exhibit characteristics consistent with apoptosis initiated during the critical interval of the SCC, including caspase activation and plasmalemmal phosphatidylserine translocation. These data suggest that decreased cell survival for injuries near the soma is due to Ca(2+)-initiated apoptosis during the critical interval of the third phase of the SCC transient. (c) 2005 Wiley-Liss, Inc.

Plasmalemmal sealing of transected mammalian neurites is a gradual process mediated by Ca(2+)-regulated proteins.

Cultured mammalian PC12 or B104 cells do not instantaneously restore a plasmalemmal barrier (seal) after neurite transection, as measured using fluorescent dye probes of various sizes and saline solutions with different [Ca(2+)](o). Rather, transected cells gradually (from 15 to 60 min postseverance) exclude probes (dye molecules) of progressively smaller size. Furthermore, an inhibitor (calpeptin) of a Ca(2+)-activated cysteine protease (calpain) and antibodies or toxins to a Ca(2+)-regulated protein (synaptotagmin) and other membrane fusion proteins (syntaxin and synaptobrevin) inhibit plasmalemmal sealing. These data obtained using molecular probes on mammalian cell lines are consistent with previous data on invertebrate giant axons indicating that Ca(2+) plays many roles in the formation, accumulation, and fusion/interaction of vesicles gradually forming a seal at a site of plasmalemmal damage.