Rapid Quantitative Fluorescence Detection of Copper Ions with Disposable Microcapsule Arrays Utilizing Functional Nucleic Acid Strategy

Abstract

In this work, an economical and easy-to-use microcapsule array fabricated by ice printing technique has been realized for ultrasensitive fluorescence quantification of copper ions employing functional nucleic acid strategy. With ice printing, the detection reagents are sealed by polystyrene (PS) film isolation and photopolymer, which guarantees a stable and contamination-free environment for functional nucleic acid reaction. Our microcapsule arrays have shown long-term stability (20 days) under -20 °C storage in frozen form before use. During the Cu²⁺ on-site detection, 1 μL sample is simply injected into the thawy microcapsule by a microliter syringe under room temperature, and after 20 minutes the fluorescence result can be obtained by an LED transilluminator. This method can realize the detection limit to 100 nM (100 fmol/μL) with high specificity.

Figure 1

Fabrication of Microcapsule arrays. (a) The fabrication system of ice printing. (b) Step-by-step procedure of fabricating microcapsule arrays in this work.

Figure 2

Main principle of Cu²⁺ fluorescence on-chip detection based on DNAzyme.

Figure 3

Fluorescence intensity of negative control with/without the PS film protection. In the inserted photo, the left microcapsule is fabricated with PS film, however, the right one isn’t. The fluorescence intensity of the microcapsule without PS film is higher than that of the microcapsule with PS film, indicating that the PS film protects the reaction solution inside the microcapsule.

Figure 4

Calibration curve between fluorescence value and Cu²⁺ concentration. The inserted photo shows that six microcapsules on one chip are separately injected six Cu²⁺ solutions whose concentration range from 0 nM, 100 nM, 200 nM, 500 nM, 1 μM to 5 μM.

Figure 5

Specificity test of Cu²⁺ fluorescence detection on the microcapsule array chip. In contrast to the Cu²⁺ concentration of 5 μM, the concentrations of Pb²⁺, Mn²⁺, Cr²⁺, and Ni²⁺ are 10 μM. Inset: Photo of the specificity test.

Figure 6

Stability test of the microcapsule array chip. The tested chips are immediately used or used after storages of 10 days, 20 days and 30 days. (a) Photos of the stability test. (b) The bar chat of measured fluorescence value, which shows the feasibility of 20-day chip storage.