Organic Nanocrystal Coated-Microfluidic Paper Analytical Device for Heavy Metal Ions Detection
Authors: Grasianto G. , Fukuyama Mao , Mott Derrick Michael , Koseki Yoshitaka , Kasai Hitoshi , Hibara Akihide 
Published: May 2020

Introduction

Monitoring heavy metal ions in the environment especially in river water is very important due to their high toxicity. Heavy metal ions can accumulate in the water and the excessive heavy metal intake become serious problem causing many negative impacts on human health such as kidney, liver, and lung damage. Many techniques are available to analyze the total amount of heavy metal ions in the water, but most techniques require high cost, samples pretreatment and expert operator, such as inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES), UV-Vis spectroscopy, X-ray fluorescence and anodic stripping voltammetry.

Low cost, simple and reliable technologies are preferred for determination of heavy metal ions without reducing analytical precision. Microfluidic paper analytical device (µPADs) is one of the most promising platforms in many fields especially in heavy metal ion analysis which satisfy these requirements. Selection of µPADs to determine heavy metal ions in river water is due to its cheapness, ease of use, and simplicity. Here, we combined µPADs with organic nanocrystal to enhance the sensitivity of heavy metal detection.

In this work, we developed for the first time organic nanocrystal coated µPADs which are effective for the determination of heavy metal ions. Simple re-precipitation method was used for generation of organic nanocrystals (NCs) composed of hydrophobic chelating agents [1,2]. NCs were coated uniformly over paper surface and could not be easily removed by employing aqueous metal-ion solution. In addition, organic nanocrystal has chemical response inherited from the chelate and is good for sorption-based color generation. The color changes of organic nanocrystal coated µPADs after addition of metal solutions were analyzed.

Method

Organic nanocrystals were prepared by injection of hydrophobic chelating agent solution into aqueous buffer [tris(hydroxymethyl)methylamino]propanesulfonic acid (TAPS) at pH 8.4 under a high speed stirring condition. µPADs were fabricated by a photolithographic method [3] with minor modification using SU-8 3050 as photoresist. NCs solution (750 µL) was introduced to µPADs with a micropipette with an injection rate of 25 µL /s, and then it was dried under desiccator vacuum for 30 minutes. Colorimetric method has been used for quantitative analysis from multiple color changes based on the L*a*b* color space [4]. A digital image of the NCs coated-μPADs was taken with a smartphone (Samsung J5) before and after exposing to a metal solution under a light box. The distance between NCs coated-μPADs and the smartphone camera was 12 cm. The L*a*b* images were obtained with ImageJ software.

Results and Discussions

NCs of three organic chelating reagents, bathophenanthroline (Bathophen), 1-(2-pyridylazo)-2-naphthol (PAN), and 1-(2-thiazolylazo)-2-naphthol (TAN) have been successfully coated onto µPADs. Scanning electron microscope (SEM) observation revealed that organic nanocrystals were not removed by exposure to excessive metal solutions. Organic nanocrystal coated-µPADs have a strong response to heavy metal ion solution at ppm level and showed different color with different heavy metal ions.

Multivariate data analysis, with 3-dye NCs color-responses to several ion species, was utilized to determine the concentration of heavy metal ions in river water. 250 µL heavy-metal-ions solution in ppb level was applied to NCs-coated µPADs. The results showed that NCs-coated µPADs is a promising device for heavy metal ions determination in river water. In the presentation, we will report the preparation method of NCs-coated µPADs and its application to heavy metal ions determination.

Acknowledgements

This work was supported by the Research Program of "Five-star Alliance" in "NJRC Mater. & Dev."

References

[1] Kasai, H. Nalwa, H. S., Oikawa, H., Okada, S., Matsuda, H., Minami, N., Kakuta, A., Ono, K., Mukoh, A., and Nakanishi, H., A Novel Preparation Method of Organic Microcrystals, Jpn. J. Appl. Phys., 31 (8), L1132-4 (1992).

[2] Takahashi,Y., Kasai,H., Nakanishi, H., and Suzuki, TM. Test Strips for Heavy-Metal Ions Fabricated from Nanosized Dye Compounds, Angew. Chem. Int. Ed. 45, 913–916 (2006) DOI: 10.1002/anie.200503015

[3] Busa, L.S.A., Maeki,M., Ishida, A., Tanib, H. and Tokeshi, M.,Simple and Sensistive Colorimetric Assay System For Horseradish Peroxidase Using Microfluidic Paper-Based Devices, Sensors and Actuators B 236 433-441 (2016). http://dx.doi.org/10.1016/j.snb.2016.06.013

[4] Komatsu,T. Mohammadi, S., Busa, L.S.A., Maeki,M., Ishida, A., Tanib, H. and Tokeshi, M. Image Analysis for a Microﬂuidic Paper-Based Analytical Device Using The CIE L*A*B* Color System, Analyst, 141, 6507 (2016) DOI: 10.1039/c6an01409g