High‐Performance Room Temperature Ammonia Sensors Based on Pure Organic Molecules Featuring B‐N Covalent Bond

Abstract Exploring organic semiconductor gas sensors with high sensitivity and selectivity is crucial for the development of sensor technology. Herein, for the first time, a promising chemiresistive organic polymer P‐BNT based on a novel π‐conjugated triarylboron building block is reported, showcasing an excellent responsivity over 30 000 (Ra/Rg) against 40 ppm of NH3, which is ≈3300 times higher than that of its B‐N organic small molecule BN‐H. More importantly, a molecular induction strategy to weaken the bond dissociation energy between polymer and NH3 caused by strong acid‐base interaction is further executed to optimize the response and recovery time. As a result, the BN‐H/P‐BNT system with rapid response and recovery times can still exhibit a high responsivity of 718, which is among the highest reported NH3 chemiresistive sensors. Supported by in situ FTIR spectroscopy and theoretical calculations, it is revealed that the N‐H fractions in BN‐H small molecule promoted the charge distribution on phenyl groups, which increases charge delocalization and is more conducive to gas adsorption in such molecular systems. Notably, these distinctive small molecules also promoted charge transfer and enhanced electron concentration of the P‐BNT sensing polymer, thus achieving superior B‐N‐containing organic molecules with excellent sensing performance.


Experimental details
General. 1 H NMR spectra were measured with a Bruker AV-400 spectrometer in CDCl 3 at 25 °C.
Chemical shifts are reported in ppm using CHCl 3 (7.26ppm) for 1 H NMR, using CDCl 3 (77.16ppm) for 13 C NMR, and using CDCl 3 for 11 B NMR as an internal standard.Thermal analysis was performed on a TG 209 instrument under nitrogen flow at a heating rate of 10 °C min -1 .The morphologies and structures of the samples were characterized by using a field emission scanning electron microscope (SEM, Quanta 250 FEG) and transmission electron microscopy (TEM) (JEM-1011; JEOL Co., Japan) operated at an accelerating voltage of 100 kV.The current (I)-voltage (V) curves were measured by a source Meter (2612B, Keithley) at room temperature.Fourier transform infrared spectroscopy (FTIR) spectra were recorded between 4000 and 650 cm −1 using a PerkinElmer Spectrum 100 FTIR spectrometer.And exposing 40 ppm ammonia gas in a vacuum environment at room temperature.

Gas sensing measurements.
Sensing device fabrication and gas-sensing performance measurements: BN-H (3.0mg), P-BNT (3.0mg), or BN-H/P-BNT (3.0mg) and dichloromethane were added to a mortar and ground for 5 minutes.Next, 10 μL solution was coated on interdigital electrodes (IDEs) to form uniform sensing films.After drying, the sensor element was aged overnight at 200 °C to improve stability.Gas sensing measurements were carried out by employing a Keithley electrometer (2611B) integrated with a customized gas testing chamber at an input signal of 3 V.The static liquid-gas distribution method was employed to calculate the volume (μL) toward the desired concentration (ppm) of test analytes.The concentration of test analytes (ammonia, acetone, triethylamine, methanol, nitrogen dioxide, and ethanol) was measured using Eq. ( 1) and injected into the evaporator inside the chamber.
where δ is the density of the analyte, Vr is the volume of analyte injected, R is the universal gas constant, T is the absolute temperature, M is the molecular weight, P b is the pressure inside the chamber and V b is the volume of the chamber.The required volume of test analytes was taken using a microliter syringe.The current of the sensor was continuously monitored until it reached the steady state in the presence of a desired concentration of test analytes.Once the steady state was reached, the chamber was exposed to the ambient atmosphere to ensure the reversibility of the sensor, and the response characteristics of the sensor were monitored continuously.The testing device and process are shown in the following figure: The response of the gas sensor (S) is defined as the ratio of resistance (Ra) of the sensor in the air to resistance (Rg) of the sensor after treatment with measured gas, that is, S = Ra/Rg.According to the least-squares method of fitting in the linear regime, the theoretical detection limit (D L ) of the gas sensor is the value of gas concentration when the sensor response is three times greater than the standard deviation of the noise signal (RMS noise ), which can be derived as follow: where N is the number of data points used in plot fitting, V  2 is standard deviation of the data points used, and the slope is the fitting plot of response versus ammonia concentration.

Synthesis
All starting materials and solvents, unless otherwise noted, were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd.All reactions were performed under an argon atmosphere.Organic solvents including CH 2 Cl 2 , THF, and toluene were dried before use.Other solvents and reagents were used without further purification unless otherwise mentioned.
( 2. Data presentation: The electrochemical measurements were independently tested three times to avoid any incidental error.The related error bars (presented in the form of mean ± SD) were also shown in the manuscript and supporting information.The sample size for each statistical analysis: The sample size of the related electrochemical measurements was three.Statistical methods used to assess significant differences with sufficient details: The statistical test was two-sided testing, the α value was 0.05, and related P values were analyzed by Student's two-sided t-test and showed in the Manuscript and Supporting Information.
3. Software used for statistical analysis: The related software were Origin, Powerpoint, and GraphPad Prism.

DFT calculations
The structural optimizations and DFT calculations of the model compounds BT, BN-H, BN-C4, and BNT-C4 were performed using Gaussian 09 program at the B3LYP/6-31G(d,p) level of theory [1] .

Figure S3 .
Figure S3.The calculated C-C bond of model compounds of BT, BN-H, BN-C4, and BNT-C4

Figure S6 .
Figure S6.a-d) Dynamic response-recovery curves of BN-H, BT, BN-C4, and BNT-C4 to 40 ppm of NH 3 at 25 °C.BN-H sensor to 40 ppm of NH 3 has the highest response value (9.56) among the four small molecules at room temperature.

Figure S11 .
Figure S11.Electrode morphology of a) BN-H, b) P-BNT, and c) BN-H/P-BNT (Inset: The photography of devices).The film thickness of 294 µm for BN-H, 268µm for P-BNT and 273 µm for BN-H/P-BNT, which can be used to calculate the conductivity.Conductivity (σ) is calculated by using the equation: σ = (I / V) × L / (w × d), where I is the current, V is the voltage, d is the thickness of the film, L and w are the length and width of the electrode.

Figure S12 .
Figure S12.The sensing mechanism of BN-H/P-BNT-based sensor to NH 3 .

Table S2 .
Summary of benchmark test results for organic ammonia sensors. S166.Morphology