<p>Black tea quality is governed by its aroma profile, which is shaped by volatile compounds that evolve across the four production stages withering, rolling, enzymatic oxidation, and firing. A reliable quality marker is trans-2-hexenal, a grassy-smelling six-carbon aldehyde that is abundant in fresh leaves but degrades during proper fermentation. Elevated residual levels in finished tea indicate under-fermentation or poor processing, whereas high-quality teas exhibit low trans-2-hexenal alongside desirable floral compounds such as linalool, geraniol, and methyl salicylate. This paper reports the development and real-world validation of a quartz crystal microbalance (QCM) sensor for selective detection of trans-2-hexenal in black tea headspace. A fluorinated polymer coating QCM-PFBBr was synthesized by functionalizing polyethylene glycol 6000 (PEG 6000) with pentafluorobenzyl bromide (PFBBr) groups and deposited onto 10 MHz AT-cut quartz crystals via electrostatic spray deposition. The sensor exhibited strong selectivity for trans-2-hexenal over key interfering volatiles, with a repeatability of 98.62%, reproducibility of 96.66%, and a Pearson correlation of r = 0.94 when benchmarked against GC–MS measurements from twenty Darjeeling tea samples. Together with previously developed QCM sensors for linalool, geraniol, and methyl salicylate, this sensor completes a four-sensor electronic nose array. The innovation of the QCM-PFBBr coating lies in its tailored π-π charge-transfer affinity for the conjugated system of trans-2-hexenal, providing a chemical specificity previously unavailable in generic polymer-based e-noses. This sensing platform offers significant application value for industrial online monitoring, enabling tea manufacturers to replace subjective organoleptic assessment with an objective, real-time tool for precise fermentation control and quality standardization. The innovation of the QCM-PFBBr coating lies in its tailored π–π charge-transfer affinity for the conjugated aldehyde system of trans-2-hexenal, providing chemical specificity unavailable in generic polymer-based sensing platforms. This four-sensor array offers significant application value for industrial online monitoring, enabling tea manufacturers to replace subjective organoleptic assessment with an objective, real-time tool for precise fermentation-endpoint control and quality standardization across production batches.</p> Graphical Abstract <p></p>

Detection of Trans-2-Hexenal using a QCM sensor in black tea

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发表日期：

2026年6月18日

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Sumana Banerjee,
Shilpi Naskar,
Prolay Sharma

摘要

Black tea quality is governed by its aroma profile, which is shaped by volatile compounds that evolve across the four production stages withering, rolling, enzymatic oxidation, and firing. A reliable quality marker is trans-2-hexenal, a grassy-smelling six-carbon aldehyde that is abundant in fresh leaves but degrades during proper fermentation. Elevated residual levels in finished tea indicate under-fermentation or poor processing, whereas high-quality teas exhibit low trans-2-hexenal alongside desirable floral compounds such as linalool, geraniol, and methyl salicylate. This paper reports the development and real-world validation of a quartz crystal microbalance (QCM) sensor for selective detection of trans-2-hexenal in black tea headspace. A fluorinated polymer coating QCM-PFBBr was synthesized by functionalizing polyethylene glycol 6000 (PEG 6000) with pentafluorobenzyl bromide (PFBBr) groups and deposited onto 10 MHz AT-cut quartz crystals via electrostatic spray deposition. The sensor exhibited strong selectivity for trans-2-hexenal over key interfering volatiles, with a repeatability of 98.62%, reproducibility of 96.66%, and a Pearson correlation of r = 0.94 when benchmarked against GC–MS measurements from twenty Darjeeling tea samples. Together with previously developed QCM sensors for linalool, geraniol, and methyl salicylate, this sensor completes a four-sensor electronic nose array. The innovation of the QCM-PFBBr coating lies in its tailored π-π charge-transfer affinity for the conjugated system of trans-2-hexenal, providing a chemical specificity previously unavailable in generic polymer-based e-noses. This sensing platform offers significant application value for industrial online monitoring, enabling tea manufacturers to replace subjective organoleptic assessment with an objective, real-time tool for precise fermentation control and quality standardization. The innovation of the QCM-PFBBr coating lies in its tailored π–π charge-transfer affinity for the conjugated aldehyde system of trans-2-hexenal, providing chemical specificity unavailable in generic polymer-based sensing platforms. This four-sensor array offers significant application value for industrial online monitoring, enabling tea manufacturers to replace subjective organoleptic assessment with an objective, real-time tool for precise fermentation-endpoint control and quality standardization across production batches.

Graphical Abstract