An optical keypad lock with high resettability based on a quantum dot-porphyrin FRET nanodevice

Abstract

Due to their appealing properties, nanomaterials have become ideal candidates for the implementation of computing systems. Herein, an optical keypad lock based on a Förster resonance energy transfer (FRET) nanodevice is developed. The nanodevice is composed of a green-emission quantum dot with a thick silica shell (gQD@SiO₂) and peripheric blue-emission quantum dots with ultrathin silica spacer (bQD@SiO₂), on which 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TSPP) is covalently linked. The nanodevice outputs dual emission-based ratiometric fluorescence, depending on the FRET efficiency of bQD-porphyrin pairs, which is highly sensitive to the metalation of TSPP: values are 59.7%, 44.8%, and 10.1% for bQD-Zn(ii)TSPP, bQD-TSPP, and bQD-Fe(iii)TSPP pairs, respectively. As such, by using the competitive chelation-induced transmetalation of TSPP, the nanodevice is capable of implementing a 3-input keypad lock that is unlocked only by the correct input order of Zn(ii) chelator, iron ions, and UV light. Interestingly, the reversible transmetalation of TSPP permits the reset (lock) operation of the keypad lock with the correct input order of ascorbic acid, Zn(ii), and UV light. Application of the nanodevice is exemplified by the construction of paper and cellular keypad locks, respectively, both of which feature signal readability and/or high resettability, showing high potential for personal information identification and bio-encryption applications.

Scheme 1. (A) Structure of the QD–TSPP FRET nanodevice. (B) Fluorescence spectra of the nanodevice with various FRET acceptors: Fe(iii)TSPP, TSPP, and Zn(ii)TSPP. (C) Illustration of the unlock and lock operations of the keypad lock device.

Fig. 1. (A) Illustration of “core-satellite” structured gQD@SiO₂/bQD@SiO₂–TSPP nanodevice. (B) TEM images of (i) gQD@SiO₂, (ii) bQD@SiO₂–TSPP and (iii) gQD@SiO₂/bQD@SiO₂–TSPP. (C) SEM images, EDX mapping and EDX spectra of (i) gQD@SiO₂/bQD@SiO₂ and (ii) gQD@SiO₂/bQD@SiO₂–TSPP.

Fig. 2. (A) Representation of the keypad lock as a network of concatenated AND gates. Nanodevice fluorescence spectra and corresponding fluorescence ratio bars for various input sequences in (B) the unlock operation and (C) the lock operation.

Fig. 3. Fluorescence ratio histogram of keypad lock unlock-lock cycles triggered by TFU and AZU input combinations. Inset images show the emission colour of the keypad lock.

Fig. 4. (A) Illustration of reversible transmetalation of TSPP based on the competitive metal chelation reaction. (B) Spectral overlaps between bQD emission and absorptions of Zn(ii)TSPP, TSPP, and Fe(iii)TSPP. (C) Absorption spectra of Zn(ii)TSPP and Fe(iii) TSPP under various chemical input sequence.

Fig. 5. (A) Structure and operation protocol of paper keypad lock. (B) Fluorescent images of the paper keypad lock during repetitive unlock-lock operations.

Fig. 6. (A) Computation of the cellular keypad lock. (B) Fluorescent (blue-channel and green-channel) images of the cellular keypad lock for the TFU and AZU input combinations. Scale bar: 10 μm. Histograms show blue-to-green emission intensity ratios of the encrypted and decrypted cellular keypad lock.