Rapid fabrication of hydrophobic/hydrophilic patterns on paper substrates for paper spray mass spectrometry

Abstract

A simple, rapid chemical coating and patterning method was developed and optimized for paper-based substrates for use in paper spray mass spectrometry (PS-MS). A variety of chlorosilanes were explored for coating paper substrates, and their effectiveness in forming hydrophobic surfaces was characterized via contact angle goniometry, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Trichloromethylsilane was selected as the primary coating agent because of the short time required to produce a hydrophobic surface (contact angle > 130°), as well as the ease of patterning. Patterning was performed using 3D-printed masks and an oxygen/plasma cleaner. Optimal mask thickness and oxygen/plasma cleaning parameters were determined to produce channels varying from 0.5 to 2.5 mm in width. The effectiveness of the patterned substrates for PS-MS was determined via analysis of four antiretrovirals: emtricitabine, lamivudine, efavirenz, and dolutegravir. Calibration curves were made for each antiretroviral at varying channel widths, and the limits of detection and limits of quantification for each drug were determined. These results show that this patterning method results in an average 7.2-fold improvement in sensitivity and an average 190-fold improvement in limits of detection over uncoated paper substrates in a neat matrix. In a proof-of-concept experiment, calibration curves were generated for each antiretroviral in urine. A patterned paper substrate with a 2-mm channel resulted in an average 7.4-fold improvement in sensitivity and an average 18-fold improvement in limits of detection over uncoated paper substrates.

Figure 1.

Contact angle as a function of coating time and silane concentration for paper substrates coated with four silanes: trichloromethyl silane (TCMS), trichlorophenylsilane (TCPhS), trichlorooctylsilane (TCOS), and trichloro-(3,3,3-trifluoropropyl)-silane (TCFS).

Figure 2.

Sample contact angle images and angles for paper substrates coated with TCPhS, TCFS, TCMS, and TCOS. Each of the images was collected after immersing the papers for 2 h in 50 mM of their respective silanes.

Figure 3.

SEM images for an uncoated paper substrate and paper substrates coated with 50 mM of each silane for two hours (top) with corresponding EDS spectra (bottom). All SEM images were collected at 300x magnification and a probe current of 38.0 with the secondary electron detector. The beam voltage was 1.5 kV for the uncoated paper and was 2.0 kV for all of the coated papers.

Figure 4.

A) Sample mask made in orange plastic. B) Paper coated with TCMS and patterned with oxygen plasma, with dye added to visualize the channels. C) Paper coated with TCOS and patterned with oxygen plasma, with dye added to visualize the channels.

Figure 5.

Measured channel width as a function of mask channel width for a 2-mm thick mask for 8 different oxygen plasma conditions from papers coated with 50 mM TCMS for 5 min. The dashed line indicated the target channel width. No channels were formed when the plasma cleaner was set at low for 5 seconds.

Figure 6.

A) 3D-printed masks made from PLA for each channel width, ranging from 0.5 mm to 2.5 mm, shown in 0.5-mm increments from left to right. The outer dimensions of the masks are 15×25 mm. B) Paper substrates corresponding to the masks above, coated with TCMS (50 mM in hexane, 5 min) and patterned via oxygen/plasma with dye added to visualize the channels. The paper dimensions are 8 × 16 mm (base x height).

Figure 7.

Calibration curves for uncoated (red squares) and patterned papers with 2-mm channels (black circles) for four target antiretrovirals in methanol/water. The y-axis is the area of the analyte peak (A) divided by the area of the internal standard peak (IS). The error bars represent the standard error of 3 measurements.

Figure 8.

Sample MS/MS spectra for each of the four antiretrovirals analyzed in the study. The spectrum for each drug is labeled accordingly: lamivudine (3TC), emtricitabine (FTC), efavirenz (EFV), and dolutegravir (DTG). Precursor and product ions for each drug are labeled in the figure.

Figure 9.

Calibration curves for uncoated (red squares) and patterned papers with 2-mm channels (black circles) for four target antiretrovirals spiked in urine. The y-axis is the area of the analyte peak (A) divided by the area of the internal standard peak (IS). The error bars represent the standard error of 3 measurements.