Daniel Citterio

Bright, color-tunable fluorescent dyes in the visible-near-infrared region.

The newly synthesized Keio Fluors, which are based on boron-dipyrromethene (BDP), have excellent and useful optical properties:  vivid colors and sharp emission in the visible−near-infrared region (583−738 nm), high quantum yields (Φ:  0.56−0.98), high extinction coefficients (185000−288000 M-1 cm-1), and good photostabilities. These optical properties are superior to many of the existing fluorescent dyes such as rhodamines, cyanines, or other BDP-based fluorescent dyes.

Paper‐Based Inkjet‐Printed Microfluidic Analytical Devices

Rapid, precise, and reproducible deposition of a broad variety of functional materials, including analytical assay reagents and biomolecules, has made inkjet printing an effective tool for the fabrication of microanalytical devices. A ubiquitous office device as simple as a standard desktop printer with its multiple ink cartridges can be used for this purpose. This Review discusses the combination of inkjet printing technology with paper as a printing substrate for the fabrication of microfluidic paper-based analytical devices (μPADs), which have developed into a fast-growing new field in analytical chemistry. After introducing the fundamentals of μPADs and inkjet printing, it touches on topics such as the microfluidic patterning of paper, tailored arrangement of materials, and functionalities achievable exclusively by the inkjet deposition of analytical assay components, before concluding with an outlook on future perspectives.

Inkjet printed (bio)chemical sensing devices

Inkjet printing has evolved from an office printing application to become an important tool in industrial mass fabrication. In parallel, this technology is increasingly used in research laboratories around the world for the fabrication of entire (bio)chemical sensing devices or single functional elements of such devices. Regularly stated characteristics of inkjet printing making it attractive to replace an alternative material deposition method are low cost, simplicity, high resolution, speed, reproducibility, flexibility, non-contact, and low amount of waste generated. With this review, we give an overview over areas of (bio)chemical sensing device development profiting from inkjet printing applications. A variety of printable functional sensor elements are introduced by examples, and the advantages and challenges of the inkjet method are pointed out. It is demonstrated that inkjet printing is already a routine tool for the fabrication of some (bio)chemical sensing devices, but also that novel applications are being continuously developed. Finally, some inherent limitations of the method and challenges for the further exploitation of this technology are pointed out.

A near-infrared fluorescent calcium probe: a new tool for intracellular multicolour Ca2+ imaging

We report a novel near-infrared fluorescent calcium probe (KFCA), which has good optical properties such as intense NIR fluorescence emission (670 nm, QY: 0.24), excellent ON/OFF ratio (120-fold), and good wavelength-compatibility with visible-light-emissive fluorophores (Fluo-4, DsRed2), and which is applicable for real-time dual-colour intracellular Ca(2+) imaging.

Inkjet printing: an integrated and green chemical approach to microfluidic paper-based analytical devices

This paper describes a new method for the fabrication of microfluidic paper-based analytical devices (μPADs) by inkjet printing alone. Microfluidic structures are patterned within less than 5 min on the surface of untreated filter paper by printing a hydrophobic UV curable acrylate composition made of non-volatile and not readily flammable compounds. After ink penetration into the paper and UV curing for 60 s, hydrophobic barriers are formed. Microfluidic channels as narrow as 272 ± 19 μm are achieved. Printed patterns retain their aqueous liquid guiding functionality for at least 6 months when stored at room temperature and for at least 72 h at 50 °C. Printing a thin uniform film on the top of the paper allows the creation of protective layers, resulting in “tunnel-like” sections of microfluidic channels entirely surrounded by inkjet printed hydrophobic barriers. Finally, the same inkjet printer is used to print reagents necessary for colorimetric sensing, which is demonstrated in the example of an enzymatic H2O2 sensor. The detection limit of the μPAD for aqueous H2O2 is 14.4 μM when applying colorimetric data processing. The only equipment required for the entire μPAD fabrication process is an off-the-shelf inkjet printer and a UV light source. This is the first application of standard printing technology for the fully integrated fabrication (microfluidic pattern, back cover, variable pattern depth, enclosed microfluidic structures, biosensor) of entire μPADs.

Reversible chemical reactions as the basis for optical sensors used to detect amines, alcohols and humidity

A new class of indicator dyes for use in analytical chemistry is presented. In contrast to most existing indicator dyes, which change colour upon complexation or protonation/deprotonation, the dyes presented here perform reversible chemical reactions with the analyte, resulting in changes in absorbance or fluorescence. Specifically, azo and stilbene dyes which exhibit a reactive trifluoroacetyl group, reversibly interact with alcohols, amines or water to give the respective diol, hemiaminal, hemiacetal or zwitterion. Indicator dyes that combine the properties of a chemical reagent and a ligand molecule are termed ‘chromo-’ or ‘fluororeactands’. They are embedded in various polymer materials and have been characterised for the optical sensing of aliphatic amines, alcohols and humidity. Furthermore, methacrylate derivatives of the dyes have been tested. They were prepared to link the dyes covalently to the polymer matrix. The optical and physical properties of the dye methacrylates make them promising candidates not only for optical sensing but also for applications in molecularly imprinted polymers and non-linear optics.

Fluororeceptor for zwitterionic form amino acids in aqueous methanol solution

A fluorescent receptor for underivatized amino acids was developed. The receptor based on triaza-18-crown-6 ether combined with two guanidinium groups could bind several biologically important amino acids in aqueous methanol solution and showed fluorescence enhancement response by a PET (photoinduced electron transfer) mechanism.

Development of chromogenic reactands for optical sensing of alcohols

Abstract A new class of chromogenic reactands (or ‘chromoreactands’) has been synthesised that reversibly interact with alcohols resulting in a change in absorbance. When embedded in plasticised PVC membranes, 4-(N,N-dioctylamino)-4′-trifluoroacetyl-azobenzene (ETHT 4001) shows a significant signal change on exposure to aqueous ethanol solution with a decrease in absorbance at around 490 nm and an increase in absorbance at around 430 nm wavelength. The sensor layer exhibits a dynamic range from 2 to 50 vol% ethanol with highest sensitivity in the 5–35 vol% range. The limit of detection is 1.5 vol%. The absorbance of the sensor membrane is virtually insensitive to changes in pH, however, the magnitude of the relative signal change between plain buffer and buffer containing ethanol is pH dependent. A similar response is observed for 1-(N,N-dioctylamino)-4-(4-trifluoroacetylphenylazo)-naphthalene (ETHT 4002), however, with the absorbance shifted to longer wavelengths. The behaviour of the reactands dissolved in alcohols correlates with the selectivity of the dyes in the sensor membranes.

Newly developed Mg2+-selective fluorescent probe enables visualization of Mg2+ dynamics in mitochondria.

Mg2+ plays important roles in numerous cellular functions. Mitochondria take part in intracellular Mg2+ regulation and the Mg2+ concentration in mitochondria affects the synthesis of ATP. However, there are few methods to observe Mg2+ in mitochondria in intact cells. Here, we have developed a novel Mg2+–selective fluorescent probe, KMG-301, that is functional in mitochondria. This probe changes its fluorescence properties solely depending on the Mg2+ concentration in mitochondria under physiologically normal conditions. Simultaneous measurements using this probe together with a probe for cytosolic Mg2+, KMG-104, enabled us to compare the dynamics of Mg2+ in the cytosol and in mitochondria. With this method, carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP)–induced Mg2+ mobilization from mitochondria to the cytosol was visualized. Although a FCCP–induced decrease in the Mg2+ concentration in mitochondria and an increase in the cytosol were observed both in differentiated PC12 cells and in hippocampal neurons, the time-courses of concentration changes varied with cell type. Moreover, the relationship between mitochondrial Mg2+ and Parkinsons disease was analyzed in a cellular model of Parkinsons disease by using the 1-methyl-4-phenylpyridinium ion (MPP+). A gradual decrease in the Mg2+ concentration in mitochondria was observed in response to MPP+ in differentiated PC12 cells. These results indicate that KMG-301 is useful for investigating Mg2+ dynamics in mitochondria. All animal procedures to obtain neurons from Wistar rats were approved by the ethical committee of Keio University (permit number is 09106-(1)).

A novel luciferin-based bright chemiluminescent probe for the detection of reactive oxygen species

This communication reports the synthesis, chemiluminescence properties, and biological application of KEIO-BODIPY-imidazopyrazine (KBI), a yellow-green chemiluminescent probe for the detection of reactive oxygen species (ROS) generated from living cells.