Editorial – in vivo analysis

It is my pleasure to introduce this themed issue of Analyst, devoted to some challenging topics within research into in vivo analysis. In vivo analysis has attracted more and more attention, with increasing numbers of published manuscripts, conference presentations, funding opportunities, and support from some scientific journals, e.g., Science, Nat. Methods, J. Am. Chem. Soc., Angew. Chem., Int. Ed., Anal. Chem., ACS Chem. Neurosci., and ACS Nano, especially after the start of the Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative^SM by the National Institutes of Health, USA. BRAIN is a USA-wide multi-year project with the aim to discover the mechanism of how the collective actions of neurons produce brain function. At its essence, neuronal function is carried out by chemicals in nature. Neurochemicals relay messages between the synapses of neurons by converting action potentials into chemical signals. Thus, the development of in vivo analysis is critical to investigating the complex problems associated with the chemical nature that underlies brain function.

To meet the challenges of in vivo analysis, we held a workshop on this topic at the Institute of Chemistry, Chinese Academy of Sciences, in October 2013, and decided to organize this themed issue. In general, analytical methods capable of in vivo monitoring of the dynamic changes of neurochemicals can fall into two categories. One is in vivo sensing and biosensing, which employs microsized electrochemical or optical probes that are implanted directly into the brain regions to record in real time the dynamic changes of neurochemicals in the central nervous system. The other is in vivo sampling, combined either with sample separation and offline detection for simultaneous multiple neurochemical analysis, or with selective online detection for continuous monitoring of single/multiple neurochemicals. The breadth of this field is represented in the 20 manuscripts in this themed issue with respect to in vivo electrochemistry, fluorescence or photoacoustic imaging, and in vivo microdialysis sampling combined with various online detections. This is a tribute to the broad interest and highly multidisciplinary nature of the field of in vivo analysis. Among them, four leading researchers in this field have reviewed recent progress from different viewpoints. Ewing and co-workers (c4an02172j) provide a snapshot of the field of in vivo analysis in invertebrates covering the recent advances in in vivo studies of chemical measurements with the focus on papers published within the last three years. They make a brief overview of the different methods and techniques used for in vivo chemical measurements in adult and larvae Drosophila melanogaster by electrochemical detection, and of other invertebrates such as Caenorhabditis elegans by fluorescence imaging. Amatore and co-workers (c4an01932f) review the amperometric detection of exocytosis in real time at the single-cell level involving microsystems/microdevices and covering the period from 2005 to 2014. In particular, they describe and discuss the most informative examples related to combinatory analysis at different scales of biology, as well as transparent materials for coupling with intracellular fluorescence techniques. Michael and co-workers (c4an02065k) review the effects of fast scan cyclic voltammetry (FSCV) and in vivo microdialysis upon the tissue next to the microelectrode or probe that is used for measuring dopamine in the surrounding brain tissue. For this, they use the ‘voltammetry next to the probe’ method with FSCV to measure the dopamine gradients in the traumatized brain tissue near the microdialysis probe. Despite many of the differences coming from the penetration injury, which is caused by implantation of the microdialysis probe into living brain tissue, good progress has been made in improving the overall health of the tissue surrounding the probe, thus helping to improve the technology of microdialysis. Paul and Stenken (c4an01898b) provide a review on a somewhat fundamental consideration on the flux issues for in vivo neurochemical measurements. They systematically review the potential effects of mass transport or flux of neurochemicals in the brain on the measurements, and offer a possible elucidation on such effects. They also exemplify these case-sensitive effects on the measurements of some neurochemicals including oxygen, nitric oxide, glucose, lactate, and catecholamines, using methods such as in vivo electrochemistry, in vivo microdialysis, and imaging techniques.

Contributions on in vivo analysis without sampling focus on a wide diversity of target neurochemicals and methods in this themed issue. Lowry et al. (c4an02027h) modify a Pt microelectrode with choline oxidase, using poly(o-phenylenediamine) for interference rejection, and stabilizing agents (methyl methacrylate, cellulose acetate, bovine serum albumin, glutaraldehyde, and polyethyleneimine), to develop a first-generation Pt-based polymer enzyme composite biosensor for the sensitive and selective detection of choline in brain extracellular fluid. In doing so, they develop an in vivo microbiosensor for the sensitive and selective detection of choline under chronic conditions, also confirming their sensor's response to extracellular choline through the significant signal change as a result of the local administration of choline and neostigmine, an acetylcholinesterase inhibitor. Zhang et al. (c4an02352h) developed an oxidase-based glucose biosensor using Prussian blue (PB)/polyaniline (PANI)/multi-walled carbon nanotubes (MWNTs) as an electrocatalyst for the reduction and determination of H₂O₂ generated from the glucose oxidase-based enzymatic catalytic reaction. The use of PB/PANI/MWNTs in this work to replace the ‘natural peroxidase’ (i.e., horseradish peroxidase) used in some existing microbiosensors enables the developed method to be facile but selective for in vivo measurements of glucose, that is virtually interference-free from ascorbic acid and other electroactive species that co-exist in the brain. Huang et al. (c4an02056a) use a self-supported nanoporous gold microelectrode modified with well-dispersed and tiny platinum nanoparticles as an electrochemical nonenzymatic biosensor for monitoring H₂O₂ at the single-cell level. FSCV offers many advantages for studying electroactive neurotransmitters because of its sensitivity, selectivity and high temporal resolution. Hashemi et al. (c5an00313j) optimize the electrochemical waveform, which enables a stimulation-locked and unique electrochemical signal for histamine. In addition to the application of electrochemical methods for in vivo analysis, fluorescence methods have also been demonstrated to be useful for such pursuits. Chen et al. (c4an02366h) designed a new BODIPY-based, turn-on, near-infrared fluorescent probe molecule BD-ss, which exhibits a high selectivity and sensitivity for H₂S_n. Jayasree et al. (c4an01507j) use autofluorescence spectroscopy, combined with multivariate statistical techniques and principal component analysis–linear discriminant analysis, for optical diagnosis of the progression and reversal of CCl₄-induced liver injury in a rodent model. Polymeric nanoparticles (ca. 250 nm diameter) containing porphyrinoid macrocycles with pre-complexed depleted uranium are introduced for in vivo analysis by Jokerst et al. (c5an00207a), and they demonstrate that this new class of nanoparticles can detect actinide cations via photoacoustic imaging.

In this themed issue, there are also several articles focusing on in vivo sampling with online or offline detection. Mao et al. (c4an02089h) demonstrate an online electrochemical system (OECS) for the selective and continuous measurement of acetylcholine through the efficient integration of in vivo microdialysis, a multi-enzyme microreactor and an electrochemical detector. Tian et al. (c4an02003k) immobilize thionine on a hydrogel electrode via one-step electrodeposition for the non-enzymatic recognition of H₂O₂. Ferapontova et al. (c4an02354d) describe an electrochemical method for the picomolar-level detection of cancer biomarker urokinase plasminogen activator (uPA) with a uPA-specific RNA aptamer. Li et al. (c4an02016b) utilize in vivo microdialysis to sample neuropeptides from the hemolymph. They determine the duration needed for collection of the microdialysis samples, enabling more comprehensive identification of neuropeptide content whilst maintaining the temporal resolution of sampling. Zhao et al. (c4an01974a) demonstrate an online and continuous approach for the in vivo measurement of bisulfide in rat brain through a modified droplet-based microfluidic system that utilises a gold nanoparticle–glutathione–fluorescein isothiocyanate probe. Lunte et al. (c4an01928h) develop a system consisting of microdialysis sampling coupled to microchip electrophoresis with electrochemical detection (MD-ME-EC) for monitoring drug metabolism in freely roaming sheep.

In this themed issue, we also have three articles focusing on the application of in vivo methods for the study of brain function. Morari and Fantin (c4an01918k) investigate the effect of N-methyl-D-aspartate (NMDA) on the striato-nigral pathway in a rat model of Parkinson's disease with multi-probe microdialysis. Zhou et al. (c4an02074j) provide a step-by-step protocol for the in vivo amperometric recording of dopamine release evoked by an action potential pattern in the striatum of dysbindin–/– mice. Ma and co-workers (c5an00110b) study the increase of serotonin in medial vestibular nuclei induced by unilateral horizontal semicircular canal occlusion with in vivo microdialysis coupled with HPLC–ECD.

We greatly appreciate all contributors of this themed issue and their achievement in this exciting field. By and large, with the quick development of brain science, we need to advocate for a large-scale multidisciplinary effort aiming at in vivo analysis. We encourage more scientists to join in this exciting and prospective field.

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