Archaea-Inspired Switchable Nanochannels for On-Demand Lithium Detection by pH Activation

With the rapid development of the lithium ion battery industry, emerging lithium (Li) enrichment in nature has attracted ever-growing attention due to the biotoxicity of high Li levels. To date, fast lithium ion (Li+) detection remains urgent but is limited by the selectivity, sensitivity, and stability of conventional technologies based on passive response processes. In nature, archaeal plasma membrane ion exchangers (NCLX_Mj) exhibit Li+-gated multi/monovalent ion transport behavior, activated by different stimuli. Inspired by NCLX_Mj, we design a pH-controlled biomimetic Li+-responsive solid-state nanochannel system for on-demand Li+ detection using 2-(2-hydroxyphenyl)benzoxazole (HPBO) units as Li+ recognition groups. Pristine HPBO is not reactive to Li+, whereas negatively charged HPBO enables specific Li+ coordination under alkaline conditions to decrease the ion exchange capacity of nanochannels. On-demand Li+ detection is achieved by monitoring the decline in currents, thereby ensuring precise and stable Li+ recognition (>0.1 mM) in the toxic range of Li+ concentration (>1.5 mM) for human beings. This work provides a new approach to constructing Li+ detection nanodevices and has potential for applications of Li-related industries and medical services.


Experimental section
a. Functionalization of nanochannels Firstly, the AAO substrates were washed with ethanol to remove the impurities during production.After drying, oxygen plasma was conducted at 200 W for 5 min to generate hydroxyl groups on AAO substrates.Then the substrates were immersed in a mixture of 2.5 ml KH560 and 50 ml methanol for 12 h to graft epoxy groups.The silanization reaction could be terminated by soaking the substrates in ethanol.We washed the substrates twice with methanol to remove the physically absorbed KH560 and then with DMF to remove the methanol.A 10 mg/mL NH 2 -HPBO ligand solution was prepared using DMF as solvent.To make the NH 2 -HPBO covalently couple to the surface, the substrates were soaked in the above solution at 80 ℃ for 24 h.The chemically modified AAO substrates were washed with DMF and then with ethanol.Finally, the substrates were cleaned by MilliQ water (18.2MΩ) and dried in the air at ambient temperature for 12 h.

b. Regulation of diameter of AAO substrates
To study how the diameter influence Li + detection performance, AAO substrates with barrier layers of different size (20-30 nm, 40-70 nm and 80-100 nm) were selected and modified to conduct I-V measurement.As shown in Scheme S1, the AAO nanochannels with diameter of 20-30 nm might be unstable in the Tris-HCl electrolyte, which could be demonstrated by the rapid current raising from Cycle 1 to Cycle 5.For the AAO nanochannels with diameter surpassing 80 nm, the current was too low to achieve high sensitivity of detection, which might be ascribed to the high mass-transfer resistance of the NH 2 -HPBO modified AAO nanochannels.

Instruments
Scanning electron microscopy (SEM) measurements were recorded in field-emission mode using a S-4800 microscope (Hitachi, Japan) with an acceleration voltage of 10 kV.

Figure S2 .
Figure S2.The digital photos of the modification and detection process of the AAO substrate, demonstrating the colorless porous AAO substrates visually turned yellow after modification.

Figure S4 .Figure S5 .
Figure S4.EDS of the modified AAO substrate showed the element distribution.

Figure S6 .Figure S7 .
Figure S6.Contact angles of the porous AAO substrates with barrier layer (top side) at every stage of 24.9 ± 1.0°, 61.3 ± 0.8°, and 68.8 ± 1.6° and photo-graphs (insets) illustrating the shape of water droplet on the porous AAO substrates with barrier layer.

Figure S8 .
Figure S8.Illustration of the homemade equipment using for current detecting experiments.

Figure S20 .
Figure S20.Schematic diagram and digital photos of the response process of NH 2 -HPBO molecule to Li + .

Figure S22 .
Figure S22.Schematic diagram and digital photos of the response process of NH 2 -HPBO molecule to K + .

Figure S23 .
Figure S23.Schematic diagram and digital photos of the response process of NH 2 -HPBO molecule to Rb + .

Figure S24 .
Figure S24.Schematic diagram and digital photos of the response process of NH 2 -HPBO molecule to Cs + .