Paper-based sorptive phases for microextraction and sensing.

The simplification of the analytical procedures, including cost-effective materials and detectors, is a current research trend. In this context, paper has been identified as a useful material thanks to its low price and high availability in different compositions (office, filter, chromatographic). Its porosity, flexibility, and planar geometry permit the design of flow-through devices compatible with most instrumental techniques. This article provides a general overview of the potential of paper, as substrate, on the simplification of analytical chemistry methodologies. The design of paper-based sorptive phases is considered in-depth, and the different functionalization strategies are described. Considering our experience in sample preparation, special attention has been paid to the use of these phases under the classical microextraction-analysis workflow, which usually includes a chromatographic separation of the analytes before their determination. However, the interest of these materials extends beyond this field as they can be easily implemented into spectroscopic and electrochemical sensors. Finally, the direct analysis of paper substrates in mass spectrometry, in the so-called paper-spray technique is also discussed. This review is more focused on presenting ideas rather than the description of specific applications to draw a general picture of the potential of these materials.

Cotton fibers functionalized with ß-cyclodextrins as selectivity enhancer for the direct infusion mass spectrometric determination of cocaine and methamphetamine in saliva samples.

Cotton is a natural material with good mechanical and chemical properties. It presents a hydrophilic surface that must be modified by the introduction of appropriate groups, if semi-polar or non-polar analytes are intended to be isolated from an aqueous matrix. In this article, cotton is modified with ß-cyclodextrins, which presents a hydrophobic cavity where target molecules can be hosted. The cavity size restricts the access of larger or less accessible molecules due to their conformation, thus improving the extraction selectivity. The modified cotton fibers are packed in a disposable syringe device where all the steps of the extraction take place. The ensuing fibers were evaluated for the extraction of several drugs providing the best results for cocaine (CO) and methamphetamine (MTA). These targets were extracted from saliva samples and finally determined by direct infusion mass spectrometry. The method provides limits of detection and quantification of 0.6 and 2 µg L-1 for CO and 0.8 and 2.7 µg L-1 for MTA, with precision values at the quantification level better than the 9% (expressed as relative standard deviation). Also, relative recoveries ranged from 115% to 111% for CO and MTA, respectively, demonstrating the applicability of the proposed method. Matrix effect, which has been statistically evaluated, is not significant, showing that the extraction selectivity can compensate the circumvent of the chromatographic separation.

Silver nanoflower-coated paper as dual substrate for surface-enhanced Raman spectroscopy and ambient pressure mass spectrometry analysis.

Paper-based analytical devices (PADs) have encountered a wealth of applications in recent years thanks to the numerous advantages of paper as a support. A silver nanoflower (AgNF) modified paper-based dual substrate for both surface-enhanced Raman spectroscopy (SERS) and ambient pressure paper spray mass spectrometry (PS-MS) was developed. AgNFs were immobilized on nylon-coated paper modified with silver and ethylenediamine. The developed substrate was characterized via scanning electron microscopy and infrared spectroscopy. The densely packed nanoscale petals of the AgNFs lead to a large number of so-called hot spots at their overlapping points, which result in an enhancement of the Raman signal. In addition, the presence of the AgNFs produces an increase in the sensitivity of the mass spectrometric analysis as compared with bare paper and nylon/Ag-coated paper. The dual substrate was evaluated for the identification and quantification of ketoprofen in aqueous standards as well as human saliva from healthy volunteers. The method enables the determination of ketoprofen with a limit of detection and limit of quantification via PS-MS of 0.023 and 0.076 mg L-1, respectively, with a relative standard deviation (RSD) of 3.4% at a concentration of 0.1 mg L-1. This dual substrate enables the simple and fast detection of ketoprofen with minimal sample preparation, providing complementary Raman and mass spectrometric information. Graphical abstract.

In-syringe dispersive micro-solid phase extraction using carbon fibres for the determination of chlorophenols in human urine by gas chromatography/mass spectrometry.

In this article, carbon fibres (CFs) are presented as sorbent material for the dispersive micro-solid phase extraction of twelve chlorophenols from urine samples. CFs are synthesized by a reagentless and green procedure consisting of heating raw cotton, a natural precursor, at high temperature (400°C) in an inert atmosphere (Ar) during 2h. The resulting fibres, which present good water dispersibility, are finally loaded on an in-syringe device. This device, which integrates the extraction and final elution of the analytes, is disposable and it is adapted to process low sample volumes. Working at the optimum conditions, the extraction procedure in combination with gas chromatography/mass spectrometry allows the determination of the analytes in urine at the low µg/L range. In fact, the limits of quantification (LOQs) of the analytes were in the interval from 1µg/L to 2.5µg/L with precision values, expressed as relative standard deviations (RSD), better than 13%. Relative recovery values, ranging from 74.5% to 113%, demonstrate the applicability of the proposed method.

Carbon coated titanium dioxide nanotubes: synthesis, characterization and potential application as sorbents in dispersive micro solid phase extraction.

In this article, carbon coated titanium dioxide nanotubes (c-TNTs) have been synthesized. The synthesis of the bare TNTs (b-TNTs) using anatase as precursor and their coating with a caramel layer have been performed by simple and cheap hydrothermal processes. The final conversion of the caramel layer in a carbon coating has been accomplished by a thermal treatment (600°C) in an inert (Ar) atmosphere. The c-TNTs have been characterized by different techniques including transmission microscopy, infrared spectroscopy, X-ray powder diffraction, thermogravimetry and Brunauer, Emmett and Teller (BET) adsorption isotherms. The extraction performance of the c-TNTs under a microextraction format has been evaluated and compared with that provided by b-TNTs and multiwalled carbon nanotubes (MWCNTs) using naproxen and ketoprofen as model analytes. c-TNTs provided better results than the other nanoparticles, especially at low acidic pH values. In addition, c-TNTs presented a better dispersibility than MWCNTs, which is very interesting for their use in dispersive micro-solid phase extraction. Finally, a microextraction format, adapted to low sample volumes, has been proposed and applied for the determination of naproxen and ketoprofen in saliva and urine samples by liquid chromatography with UV detection. The results indicate that this approach is promising for the analysis of biological samples. In fact, the recoveries were in the range between 96% and 119% while the precision, expressed as relative standard deviation, was better than 8.5% and 26.3% for urine and saliva, respectively. The detection limits were in the range 34.1-40.8µg/L for saliva samples and 81.1-110µg/L for urine samples.