Microfluidic paper-based analytical devices (µPADs) have emerged as a promising alternative to traditional microfluidic chips due to their simplicity, low cost, portability, and disposability. Existing µPAD fabrication methods include photolithography, wax printing, paper cutting, drawing, inkjet printing, laser cutting, and stamping. Each method presents trade-offs between resolution, cost, and ease of fabrication. Photolithography offers high resolution but is complex and expensive. Wax printing is inexpensive and easy but lacks resolution. Inkjet printing provides high resolution but requires specialized printers. Laser cutting achieves the highest resolution but is complex and requires specialized paper. Stamping techniques, while generally less expensive and easier to perform, often suffer from lower resolution. Atom stamp printing (ASP), using machine-engraved penetrating stamps, is proposed as a new method to overcome these limitations. ASP leverages the microporous structure of the stamps to absorb ink for simple and high-resolution printing. This study explores ASP as a fabrication method for µPADs and demonstrates its application in detecting heavy metal ions, specifically Cu²⁺, using both colorimetric and distance-based detection methods. Furthermore, a novel paper-based solid-liquid extraction device (PSED) is developed to improve the efficiency and cost-effectiveness of heavy metal ion extraction from soil samples, addressing the need for simple and portable solutions for point-of-care testing (POCT) in resource-constrained environments. Existing heavy metal ion detection methods based on µPADs include colorimetric, fluorescence, and electrochemical methods, each with its own advantages and disadvantages. This work focuses on the colorimetric method for its simplicity and ease of use, complemented by the more accurate distance-based detection method for quantitative analysis. The integration of a 3D µPAD with a homemade micropump in the PSED allows for efficient solid-liquid extraction and filtration, bypassing the need for complex equipment such as centrifuges and ultrasonic vibrators typically required in traditional methods.

The literature review extensively covers various existing µPAD fabrication methods, including photolithography (Martinez et al.), wax printing (Lu et al., Carrilho et al.), paper cutting, drawing, inkjet printing (Abe et al.), laser cutting, and different stamping techniques utilizing various materials like PDMS and flash foam stamps (FFS). The review highlights the advantages and disadvantages of each technique, emphasizing the resolution, cost, and complexity of each method. Additionally, the review explores existing methods for heavy metal ion detection using µPADs, covering colorimetric, fluorescence (Fu et al.), and electrochemical detection methods (Hu et al.), comparing their respective advantages and disadvantages in terms of accuracy, ease of use, and cost. Specific examples of prior work utilizing colorimetric (Xu et al.), fluorescence, and electrochemical detection methods are detailed, emphasizing the motivation for developing a more cost-effective and simple method for heavy metal detection.

The study first evaluates the resolution of µPADs fabricated using ASP. Hydrophilic channels and hydrophobic barriers of varying theoretical widths were designed and printed onto filter paper using PDMS as a hydrophobic material. The actual widths were then measured under a microscope, revealing the minimum achievable widths for both hydrophilic channels (328 µm) and hydrophobic barriers (312 µm). The relationship between theoretical and actual widths was found to be approximately proportional, with high correlation coefficients. An economic analysis compares the cost of µPAD fabrication using ASP with the Flash Foam Stamp Lithography (FFSL) method, highlighting the lower cost and higher efficiency of ASP. For Cu²⁺ detection, two methods were employed: a colorimetric method using a specific reagent (not specified but implied to be a colorimetric indicator), and a distance-based detection method. The colorimetric method involved visually observing the color change of the solution in the µPADs at different Cu²⁺ concentrations and analyzing the grayscale values of the images using ImageJ software. The distance-based method measured the distance the colored solution traveled within the µPADs, which is directly related to Cu²⁺ concentration. To demonstrate the versatility of the µPADs, a paper-based solid-liquid extraction device (PSED) was developed. The PSED is a 3D µPAD with a "3+2" structure and a recyclable extraction mode, utilizing a homemade micropump to drive the extraction process. Soil samples were placed on the device and mixed with an extraction solvent, and the micropump facilitated multiple cycles of extraction and filtration. The extracted heavy metal ions were then detected using the methods described above. The efficiency of the PSED was compared to traditional solid-liquid extraction methods.

The ASP method demonstrated high resolution in µPAD fabrication, achieving minimum widths of 328 µm for hydrophilic channels and 312 µm for hydrophobic barriers. This resolution is superior to other low-cost methods like wax printing and FFSL. The economic analysis confirmed that the ASP method is significantly more cost-effective than FFSL. The colorimetric method allowed for semi-quantitative detection of Cu²⁺, with a clear color change observable visually, which was further quantified through grayscale analysis. The distance-based method provided a more accurate quantitative measurement of Cu²⁺ concentration, achieving a detection limit of 1 mg/L, meeting or exceeding regulatory limits set by the World Health Organization and the United States Environmental Protection Agency for drinking water. The PSED effectively extracted heavy metal ions (Pb, Cd, Cu, Zn) from soil samples, with results comparable to traditional methods, demonstrating its efficiency and practicality. The optimal solid-liquid ratio for extraction using the PSED was determined to be 1:10, and a minimum volume of 30 mL of extractant was found necessary for effective extraction and detection.

The results demonstrate the success of ASP as a high-resolution, low-cost, and efficient method for µPAD fabrication. The combination of colorimetric and distance-based detection methods provides both qualitative and quantitative analysis of Cu²⁺, improving the accuracy and reliability of the detection. The development of the PSED offers a significant advancement in solid-liquid extraction for heavy metal ions, simplifying the process and making it more portable and accessible, particularly valuable in resource-limited settings. The comparable performance of the PSED to traditional methods validates its effectiveness and potential for practical applications. The findings underscore the potential of integrating simple, inexpensive, and portable devices for rapid and accurate heavy metal detection in environmental monitoring and food safety applications.

This study successfully demonstrated the efficacy of atom stamp printing for high-resolution µPAD fabrication and its application in heavy metal ion detection. The developed PSED provides a simplified and cost-effective approach to solid-liquid extraction, surpassing traditional methods in terms of portability and ease of use. Future research could explore the application of ASP to fabricate µPADs with more complex designs for multi-analyte detection and further optimization of the PSED for various soil types and extraction conditions.

The study primarily focused on the detection of Cu²⁺. Further research is needed to assess the applicability of the ASP method and the PSED to other heavy metal ions and environmental matrices. The homemade micropump used in the PSED could be further improved for better control and precision. The study did not thoroughly investigate the effects of various soil properties on the efficiency of heavy metal extraction using the PSED.