Detection and extraction of heavy metal ions using paper-based analytical devices fabricated via atom stamp printing

This study addresses the need for low-cost, easy-to-fabricate, portable analytical platforms for point-of-care testing of heavy metals. Microfluidic paper-based analytical devices (µPADs) guide fluid through hydrophilic channels separated by hydrophobic barriers and can replace traditional glass or silicon microfluidic chips. Existing µPAD fabrication methods (photolithography, wax printing, cutting, drawing, inkjet printing, laser cutting, and stamping) each face trade-offs in resolution, cost, complexity, and throughput. The authors propose atom stamp printing (ASP) as a new, efficient, low-cost stamping approach using machine-engraved atom stamps and PDMS as hydrophobizing ink to pattern paper with high resolution. The research aims are to: (1) develop and characterize ASP for µPAD fabrication, including resolution and cost; (2) demonstrate detection of Cu²⁺ using colorimetric and distance-based readouts to achieve quantitative performance; and (3) introduce a paper-based solid-liquid extraction device (PSED) integrating a 3D µPAD and a micropump to extract heavy metal ions from soil as a low-cost alternative to conventional extraction that requires specialized equipment.

Prior work has established multiple µPAD fabrication strategies: photolithography with SU-8 can yield high resolution but is costly and complex; wax printing and inkjet printing are rapid and inexpensive but require special printers and typically offer moderate resolution; simple drawing and cutting are low-cost but low-resolution; laser cutting achieves the highest resolution (down to ~62 µm) but needs special coated papers and complex operation. Stamping approaches (paper/tape stamps, PDMS high-relief, iron/steel, flash foam stamps) vary in cost and resolution; flash foam stamps can use PDMS as hydrophobe with relatively better resolution but require prolonged soaking, have limited print cycles per soak, and lower throughput. For detection of heavy metals on µPADs, colorimetric methods are common and simple but often semi-quantitative; electrochemical methods provide quantitative accuracy but require instrumentation; fluorescence offers high selectivity and sensitivity but needs special reagents and instruments. Distance-based readouts on µPADs have been shown to enable quantitative measurement of analytes, including metals like Ni and Cu, providing an attractive alternative to intensity-based colorimetry.

- ASP fabrication of µPADs: Atom stamps (manually or laser-engraved) of the desired pattern were soaked in PDMS solvent and then printed onto filter paper to create hydrophobic barriers defining hydrophilic channels. Printed papers were cured in a vacuum drying box or oven. The ASP process leverages the microporous stamp to absorb and transfer PDMS efficiently. - Resolution testing: Test patterns with nominal hydrophilic channel widths (0.3–1.3 mm) and hydrophobic barrier widths (0.2–1.1 mm) were printed. Blue dye was flowed to visualize channeling and barrier performance. Actual widths were measured microscopically and compared to theoretical values to assess resolution and proportionality. - Economic analysis: Material and process costs for ASP were itemized and compared with flash foam stamp lithography (FFSL), considering consumables (filter paper, PDMS, electricity) and per-stamp costs, noting the reusability of atom stamps and high throughput of laser engraving (~20 stamps per minute). - Colorimetric detection of Cu²⁺: µPAD detection zones were functionalized to form a yellow complex with Cu²⁺ (using DDTC reagent). Cu²⁺ standards produced color changes from white to yellow with increasing intensity at higher concentrations. Images were analyzed using ImageJ to extract grayscale values (0–255) versus concentration. - Distance-based detection of Cu²⁺: A channel-based format produced a colored band whose length increased with Cu²⁺ concentration. Band lengths were measured across a range of concentrations to determine linear dynamic range, upper limit (saturation), and detection limit. - PSED for soil extraction: A 3D µPAD with a “3+2” structure was integrated with a homemade micropump to drive a recyclable extraction flow. Soil samples were placed atop the 3D µPAD. Extraction solvent was dispensed from the pump outlet to mix with soil, solubilize metals (e.g., Cu, Zn, Cd, Pb), percolate through paper for filtration, and be recirculated via the pump inlet to form extraction cycles. Extracted solutions were collected and analyzed. Solid-liquid ratios (1:5, 1:10, 1:15, 1:20) were evaluated for accuracy (error rate relative to traditional extraction), and guidance on extractant volume (>30 mL) was established to account for losses and enable downstream detection. Results from PSED were compared with traditional extraction methods to quantify agreement (error rate).

- ASP resolution: Minimum actual widths achieved were 328 µm for hydrophilic channels and 312 µm for hydrophobic barriers. Actual-to-theoretical widths showed strong proportionality (R² ≈ 0.9965 for channels; R² ≈ 0.999 for barriers). Channels conducted flow at ≥0.4 mm nominal width; barriers blocked at 0.2 mm nominal width. - Comparative performance: ASP provided higher resolution than FFSL (632 ± 27 µm channels, 306 ± 20 µm barriers) and avoided specialized equipment required for inkjet or wax printing. Although laser cutting attains finer features (~62 µm), it requires special papers and complex processing. - Cost: Estimated per-device material/process cost for ASP was ~¥0.59 (about $0.09), lower than FFSL at ~¥0.91 (about $0.15). Atom stamps are reusable; laser engraving can produce ~20 stamps per minute. - Colorimetric Cu²⁺ detection: The DDTC-Cu complex color deepened with concentration; grayscale decreased monotonically with increased Cu²⁺ (255 = white, 0 = black), enabling semi-quantitative analysis. - Distance-based Cu²⁺ detection: Band length increased with concentration and was linear up to 100 mg/L; above that, length saturated (defining the upper limit). The detection limit was 1 mg/L, matching stringent drinking water guidelines (EPA 1.3 mg/L; WHO 2 mg/L as references). - PSED extraction: Concentrations of heavy metal ions (Pb, Cd, Cu, Zn) extracted with PSED closely matched traditional methods, with error rates of 0.02–0.05 (considered acceptable; <0.05 deemed qualified). Optimal solid-liquid ratio was 1:10; ratios of 1:15 and 1:20 yielded higher error rates (0.06 and 0.09). More than 30 mL extractant volume was recommended to offset losses and facilitate detection.

The results demonstrate that atom stamp printing is a practical, low-cost, and scalable method to fabricate µPADs with sub-350 µm features, adequate for many POCT applications without the complexity of laser cutting or the equipment constraints of wax/inkjet printing. The strong linear correspondence between designed and realized dimensions supports predictable device design using ASP despite PDMS diffusion. For Cu²⁺ analysis, combining colorimetric chemistry with a distance-based readout converts subjective color intensity into an objective length measurement, improving quantitation and achieving a 1 mg/L detection limit, suitable for regulatory-relevant screening. The integrated PSED showcases how paper’s intrinsic capillarity and filterability, coupled with a simple micropump, can replicate multi-step solid-liquid extraction without centrifuges or ultrasonic equipment. Its extraction accuracy comparable to traditional methods, along with acceptable error rates and practical operating parameters (solid-liquid ratio ~1:10, extractant volume >30 mL), indicates strong potential for on-site environmental and food safety monitoring in resource-limited settings.

This work introduces atom stamp printing as an efficient, low-cost approach to fabricate µPADs with high resolution (minimum channel and barrier widths of 328 µm and 312 µm). Using these devices, Cu²⁺ detection was demonstrated via colorimetry and quantified with a distance-based readout, reaching a detection limit of 1 mg/L and a linear range up to 100 mg/L. An integrated paper-based solid-liquid extraction device leveraging a 3D µPAD and a micropump achieved extraction of heavy metal ions from soil with accuracy comparable to conventional methods, while simplifying equipment needs and reducing cost. These advances support practical POCT for environmental and food safety applications. Future work could extend ASP-fabricated µPADs and the PSED platform to additional analytes, optimize chemistries for lower detection limits, broaden linear ranges, and further integrate sample preparation and detection into all-paper systems.

- The distance-based Cu²⁺ assay exhibits band-length saturation above 100 mg/L, limiting the upper quantitation range without dilution. - PDMS diffusion during printing causes actual feature sizes to deviate from designs, requiring calibration (though deviations are proportional). - Colorimetric intensity measurements alone are semi-quantitative; the study mitigates this by employing distance-based readouts. - The PSED requires sufficient extractant volume (>30 mL) to compensate for losses in paper/soil and to enable measurement, which may constrain use when sample or reagent volumes are limited. - Higher solid-liquid ratios (1:15, 1:20) increased extraction error rates (0.06–0.09), narrowing the optimal operating window. - Although the per-device cost is low, initial access to a laser engraving machine is needed to rapidly produce high-quality atom stamps.