用于指示牛奶新鲜度的具有高且稳定的响应性的原位pH敏感木基智能标签
An In-situ pH-sensitive Wood-based Intelligent Label with High and Stable Responsiveness for Indicating Milk Freshness
[目的]解决现有的pH比色指示器制备复杂、成本高、颜色观察受干扰的问题。[方法]本研究以改性速生杨木(Populus L.)单板作为直接基质。采用自上向下的木质素保留策略，通过接枝季铵基团和吸附BTB阴离子，使温和的碱润胀和阳离子化协同作用，从而开发出一种新型智能木基标签。[结果]当 NaOH 的浓度为 1.5%、固液比为 1:30 时，单板的增重率增加到 7.03%，而孔隙率仅降低了 1.75%，证明了良好的尺寸稳定性。BTB 的吸附容量达到 8.504 mg/g，是原始单板的 38.30 倍，b*达到最低值-23.13.由于对BTB的牢固锚定和超高吸附容量，该标签具有高且稳定的pH敏感性，良好的醋酸气体响应性。当鲜牛奶的可滴定酸度上升到 20.63 °T时，(市售鲜牛奶的酸度一般要求不超过 20 °T)，pH值仅发生微小变化，而标签的颜色由蓝色变为绿色。[结论]本研究制备了一种安全且稳定的智能标签，具有醒目的pH传感功能，可用于追踪牛奶的新鲜度。抛开传统的成膜基质和方法，选择了廉价、稳定、不透明的速生长杨木单板作为直接支撑，并将其阳离子化以锚定BTB.与通常的脱木素和漂白工艺相比，这种方法保留了绝大部分的木质素，而且吸附BTB后可以忽略木材原色的干扰。季铵基团的接枝率受固液比和 Na OH 浓度的影响，其对单板的尺寸稳定性几乎没有影响。FTIR和XPS结果证明了季铵化改性的成功。SE M图像显示BTB 离子被均匀地吸附在单板的孔隙和内壁上，没有发生聚集。XRD结果表明BTB离子的吸附对纤维素的晶体结构没有影响。大量带正电的季铵基团接枝到木材纤维上，通过静电作用和错综复杂的分级孔隙，大大提高了BTB的吸附容量并抑制了其释放，在50%或 95%的乙醇水溶液(含油脂食物模拟物)中几乎没有迁移。BV在长保质期内具有优异的pH敏感性，且对CH3COOH有明显的色差响应，有望用于监测酸性变质。当应用于追踪牛奶新鲜度时，它能简单、及时、准确地指示牛奶变质，颜色在接近可食用阈值时由蓝色变为绿色，使变质信息可视化。综上所述，本研究提出了一种无需脱木素和漂白的pH 传感木基智能标签的制备方法，它能以明显的颜色响应告知食品质量信息，并将劣质木材的使用拓展到了一个新的领域。
[Objective]The aim of this study was to solve the problems of complex preparation,high cost,and interference in color observation of developed pH-colorimetric indicators.[Method]Here,modified fast-growing poplar(Populus L.)veneer was used as a direct substrate.A top-down lignin retention strategy was accomplished wherein mild alkali swelling and cationization were conducted synergically to develop a novel intelligent wood-based label via grafting quaternary ammonium groups and adsorbing BTB anions.[Result]When the concentration of NaOH was 1.5%at the solid/liquid ratio of 1:30,the weight gain rate of the veneer increased to 7.03%,while the porosity decreased by only 1.75%,proving good dimensional stability.The adsorption capacity of BTB reached 8.504 mg/g,which was 38.30 times higher than that of the original veneer,and the b* achieved the lowest value of-23.13.Due to the ultra-high adsorption capacity of securely anchored BTB,the label showed high and stable pH-sensitivity and a favorable response to acetic acid gas.Its color varied from blue to green when the titratable acidity of fresh milk rose to 20.63 °T(The acidity of commercially available fresh milk in China is generally required not to exceed 20 °T),accompanied by a tiny pH change.[Conclusion]A safe and stable intelligent label with arresting pH sensing was fabricated to track milk freshness.Eschewing conventional film-forming substrates and methods,cheap,stable,and opaque fast-growing poplar veneers were selected as the direct supports and were cationized to anchor BTB.This method retained the vast majority of lignin compared to the usual delignification and bleaching processes,and the interference of the original color of the wood could be ignored after the adsorption of BTB.The grafting rate of quaternary ammonium groups was affected by the solid-liquid ratio and NaOH concentration,which had little effect on the dimensional stability of the veneer.FTIR and XPS results proved the success of the quaternization modification.SEM images showed that BTB ions were adsorbed homogeneously on the pores and inner walls of the veneer without aggregation.XRD results suggested that the adsorption of BTB ions had no effect on the crystal structure of cellulose.A large number of positive quaternary ammonium groups grafted onto the wood fibers significantly enhanced the adsorption capacity of BTB and inhibited its release via electrostatic interactions and intricate hierarchical pores,and there was almost no migration in 50%or 95%aqueous ethanol solutions(containing fatty food simulants).The excellent pH sensitivity of BV over a long shelf life and a remarkable color difference in response to CH3COOH make it possible to monitor acidic spoilage.When applied to track milk freshness,it indicated spoilage easily,timely and accurately with color changed from blue to green near the edible threshold,visualizing spoilage information.In summary,this study proposed a method to prepare a pH-sensing wood-based intelligent label without delignification and bleaching,which informed food quality information with apparent color response and expanded the use of low-quality wood to a new area.
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