三苯基膦(PPh3)是一种重要的有机化合物，已被广泛用于Witting反应[1]、Mitsunobu反应[2]、Mukaiyama-Corey内脂化反应[3]、Appel反应[4]、Staudinger反应[5]等。由于与过渡金属之间很强的键合能力，三苯基膦常被作为配体用于碳-碳键构建，如Suzuki、Heck及Negishi偶联反应[6]。三苯基膦的广泛使用必然会对环境造成磷污染，进而威胁人类健康。已报道的检测PPh3的方法有分光光度法[7]、高效液相法[8]、氧化还原滴定法[9]、重量分析法、直接滴定法[10]等，但以上方法操作冗杂，耗时费力，所以急需一个方便、高效检测PPh3的方法。最近，荧光探针法因其具有简便性、高灵敏性和专一性的特点，已成为现代科学分析方法中一种强有力的分析工具[11-17]。已报道的荧光探针可以检测各种金属阳离子、阴离子、氨基酸和中性小分子[18-31]。最近有报道利用与PPh3发生还原反应来区分检测GSNO或HNO[32-35]，我们从中得到启发，尝试设计一个检测PPh3的探针分子。众所周知，在生物标记方法中，芳基磷化物可以和有机叠氮化物发生Staudinger连接反应(Staudinger ligation)，选择性形成酰胺键，因而广泛用于化学生物学研究[36-42]。无痕Staudinger连接反应会释放出醇或酚，因此我们考虑利用叠氮基团反过来仿效Staudinger连接反应区分检测PPh3。近几年，我们报道了一系列检测有机物的荧光探针[43-47]，本文设计的一个基于ESIPT机制、通过连接邻叠氮苯乙酸酯基团来检测PPh3的荧光增强型荧光探针分子，不仅可以在水溶液中, 也可以在滤纸上实现对PPh3的灵敏性检测。
1 实验部分 1.1 原料与仪器所用原料均购于商用厂家并没有进一步纯化，有机溶剂用标准法进一步纯化，检测中使用色谱纯乙腈和去离子水。核磁中氢谱和碳谱利用VARIAN核磁共振仪测得；低分辩质谱和高分辨质谱分别利用Agilent 1100 Series LC/MSD和AB SCIEX Triple TOFTM 5600+质谱仪测得；荧光光谱利用F7000荧光分光光度计测得。
1.2 合成路线探针1的合成路线如图 1所示。利用文献报道的方法[53]，3-羟基黄酮由邻羟基苯乙酮和4-二甲氨基苯甲醛制备得到。
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图 1 探针1的合成 Fig.1 Synthesis of probe 1 |
2-硝基苯乙酸(5.00 g, 27.6 mmol)、碳酸钾(4.00 g, 28.9 mmol)和钯碳(10%, 350 mg, 2.96 mmol)溶解于60 mL乙醇中，然后通入氢气(3000 kPa)，室温下搅拌过夜，冰水淬灭，析出的固体经抽滤去除，滤液用二氯甲烷萃取(3×100 mL)，无水硫酸钠干燥，旋干有机相，得白色固体邻氨基苯乙酸(3.44 g, 82%)。
将邻氨基苯乙酸(500 mg, 3.31 mmol, 1 equiv)溶于0.8 mL氢氧化钠(132 mg, 3.31 mmol, 1 equiv)水溶液中，将溶液逐滴加入至预冷的12 mL亚硝酸钠(228 mg, 3.31 mmol, 1 equiv)水溶液中，然后将该混合物滴加至5 ℃的12 mL硫酸溶液，滴加时间控制在10 min内。滴加完毕后，将2 mL的NaN3(215 mg, 3.31 mmol, 1 equiv)水溶液逐滴加入上述反应液中，保持在0 ℃下继续搅拌1 h，加入大量冰水淬灭，二氯甲烷萃取，水洗，无水硫酸钠干燥，得土黄色固体(360 mg, 61%)。1HNMR (400 MHz, DMSO):12.35 (s, 1 H), 7.37 (m, 1 H), 7.28 (dd, 2 H, J = 1.2 Hz, 8.0 Hz), 7.14 (m, 1 H), 3.53 (s, 2 H)。
1.2.2 探针1的合成将化合物3-羟基黄酮(50 mg, 0.21 mmol)、对二甲氨基吡啶(DMAP，5 mg, 0.01 mmol)和邻叠氮苯乙酸(44.5 mg, 0.25 mmol)溶于二氯甲烷中，冰浴下加入二环己基碳化二亚胺(DCC, 65 mg, 0.315 mmol)的二氯甲烷溶液，反应在氮气环境中, 保持冰浴冷却并搅拌15 min，然后室温搅拌12 h。反应液用蒸馏水和盐水洗涤，二氯甲烷萃取，无水硫酸钠干燥，减压蒸馏溶剂，硅胶柱层析(200~300目)分离产物(PE：EA = 1：1)，得到白色固体(49 mg, 59%)。
1HNMR (400 MHz, CDCl3):8.27 (dd, 1H, J =1.6 Hz, 8.0 Hz), 7.80 (m, 2H), 7.71 (m, 1H), 7.53 (m, 2H), 7.44 (m, 3H), 7.36 (m, 2H), 7.14 (m, 2H), 3.93 (s, 2H)。13CNMR (100 MHz, CDCl3):172.2, 168.2, 156.5, 155.8, 138.8, 134.1, 133.9, 132.0, 131.4, 130.0, 129.1, 128.7, 128.6, 126.2, 125.3, 125.1, 124.9, 123.8, 118.2, 36.5。HRMS (ESI) Calculated for C23H15N3O4 [M+H]+:398.1133; found: 398.1135。
2 结果与讨论相比于其它荧光物质，具有激发态分子内质子转移(ESIPT)特性的染料有独特的性质，如较大的Stokes位移、双发射波长、质子快速转移、四能级光循环过程等。3-羟基黄酮是一种典型的ESIPT机制的荧光染料[48-51]，近年来逐渐被应用于荧光探针和传感方面的分析检测，吸引了很多科研工作者的关注[48-56]。我们认为，三苯基膦响应基团通过与3-羟基黄酮成酯掩蔽荧光团，阻碍了ESIPT过程的发生，使得3-羟基黄酮母体荧光减弱，与三苯基膦反应后，释放荧光团, 从而对其识别。为此, 我们设计了一个检测反应机制类似于无痕Staudinger连接反应的荧光探针(Probe 1), 如图 2。该探针包含捕获三苯基膦的芳基叠氮基团，以及邻位的乙酯基团，通过类Staudinger连接，发生分子内的内酰胺化，释放3-羟基黄酮母体，恢复ESIPT过程，使荧光得到增强。初步检测发现其具有以下几个优点：制备过程简单、光稳定性好、斯托克斯位移较大(＞130 nm)。检测结果表明探针1有望实现比色法检测低浓度的PPh3。
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图 2 (a)探针1的检测机制; (b)3-羟基黄酮ESIPT作用过程的缩略图 Fig.2 (a) The proposed sensing mechanism of probe 1; (b) Schematic representation of ESIPT process of 3-Hydroxyflavone |
我们首先测试了探针1与PPh3反应后的时间响应情况。在乙腈/PBS缓冲溶液(1/1, V/V, 10 mmol/L, pH 7.3, 25 ℃)体系中，10 μmol/L探针1与100 μmol/L PPh3反应60 min。从图 3a中可看出，探针的最大吸收波长是290 nm，当100 μmol/L PPh3与探针1反应完成后，紫外吸收波长发生红移产生了两个新峰，分别红移到306 nm和344 nm；从图 3b可看出，当100 μmol/L PPh3与探针1混合后，同时出现荧光发射在411 nm和522 nm的两个发射波带，发出黄绿色荧光，这是3-羟基黄酮在烯醇和酮式两个结构之间互变异构的结果。随着时间延长，荧光强度在56 min时达到最大值(图 3c)。证明探针1与PPh3反应后有明显的荧光增强。
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图 3 (a)探针1 (10 μmol/L)与PPh3(100 μmol/L)反应60 min前后的紫外吸收光谱；(b)探针1 (10 μmol/L)与PPh3(100 μmol/L)反应60 min后的荧光光谱；(c) 522 nm处荧光强度随时间变化曲线(λex = 370 nm, Slit = 5 nm/5 nm) Fig.3 (a) Absorption spectra of probe 1 prior to and 60 min after addition of 100 μmol/L PPh3; (b) fluorescence spectral changes and (c) fluorescence intensities (λem = 522 nm)of probe 1 (10 μmol/L) in the presence of 100 μmol/L PPh3 (λex = 370 nm, Slit = 5 nm/5 nm) |
随后，我们研究了探针1随PPh3当量变化的荧光响应情况。在乙腈/PBS缓冲溶液(1/1, V/V, 10 mmol/L, pH 7.4, 25 ℃)测试体系中，我们测试了10 μmol/L探针1与不同浓度的PPh3的反应情况，浓度分别为1、2、4、6、8、10、20、40、60、80、100、150、200、250、300、400、500 μmol/L，反应时间均为60 min，测试结果如图 4。由图可知，随浓度的增加，探针1与PPh3反应速度越来越快，522 nm处的荧光强度逐渐增强。当PPh3达到300 μmol/L时，522 nm处荧光强度达到最大。其中，PPh3浓度在2~10 μmol/L范围内，522 nm处荧光发射强度与浓度呈较好的线性关系，其线性公式为Y=33.498+54.538X (R2 = 0.992)。经计算得出探针1的检测极限小于1 μmol/L(S/N=3)。因此，探针在2~10 μmol/L范围内可进行定量检测PPh3，而其它检测方法，很难精确检测如此低浓度的PPh3。
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图 4 (a)探针1 (10 μmol/L)与不同浓度PPh3(1~500 μmol/L)反应后的荧光光谱图(λex = 370 nm, Slit = 5 nm/5 nm)；(b)522 nm处荧光强度与PPh3浓度的线性关系 Fig.4 (a) Fluorescence spectra changes of probe 1 (10 μmol/L) in the presence of increasing concentrations of PPh3 (1-500 μmol/L) in acetonitrile/PBS buffer (1:1, V/V, 10 mmol/L, pH 7.3, 25 ℃) recorded after 60 min (λex = 370 nm, Slit = 5 nm/5 nm); (b) linear relationship of fluorescence enhancement at 522 nm |
为了验证探针1对PPh3的选择性，我们考察了15种常见的还原剂、亲核性物质、阴离子等物质(S2-、HS-、I-、Br-、F-、N3-、SO32-、S2O52-、HSO3-、GSH、Cys、Hcy、ClO4-、S2O42-、CO32-)对探针1在检测PPh3时的干扰。在乙腈/PBS缓冲溶液(1/1, V/V, 10 mmol/L, pH 7.3, 25 ℃)测试体系中，10 μmol/L探针1分别与100 μmol/L PPh3或1 mmol/L其它检测物反应，反应时间均为60 min，其荧光光谱如图 5所示。其中，探针1与可能的干扰物混合60 min后的荧光并未有明显增强，而探针1与PPh3混合60 min后荧光增强显著。柱状图更能清晰地反应出这种差别(图 6)。如图 6所示，黑线代表探针1与可能干扰物混合后60 min后在522 nm处的荧光强度，红线则代表探针1与PPh3及可能干扰物共存时60 min后的荧光强度，显示探针1只对PPh3有明显响应，其它物质均没有明显干扰。上述实验表明探针1对PPh3的响应具有专一性。
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图 5 探针1 (10 μmol/L)对PPh3(100 μmol/L)及其他物质(1 mmol/L)作用后的荧光光谱图 Fig.5 The fluorescent spectra of probe 1 (10 μmol/L) toward PPh3 (100 μmol/L) and other analytes (1 mmol/L), respectively (λex = 370 nm, Slit = 5 nm/5 nm) |
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图 6 探针1 (10 μmol/L)与各种检测物(100 μmol/L)反应后522 nm处的荧光强 (黑色为探针+干扰物；红色为探针+ PPh3 +干扰物) Fig.6 The fluorescence intensities (λem = 522 nm) of 10 μmol/L probe 1 in the absence and presence of 100 μmol/L of PPh3 with 1 mmol/L of other analyte (black bar, probe 1 + anion; red bar, probe 1 + anion + individual analyte) (1) Na2S, (2) NaHS, (3) KI, (4) NaBr, (5) NaF, (6) NaN3, (7) Na2SO3, (8) Na2S2O5, (9) NaHSO3, (10) GSH, (11) Cys, (12) Hcy, (13) KClO4, (14) Na2S2O4, (15) Na2CO3, (16) PPh3 (λex = 370 nm, Slit = 5 nm/5 nm) |
为研究探针受pH的影响情况，对探针1进行了pH稳定性测试。如图 7所示，pH在5~10之间时，在乙腈/水体系中，探针1 (10 μmol/L)能稳定存在，而当加入PPh3 (100 μmol/L)，探针1对PPh3有显著响应(反应时间为60 min)，且在较高pH条件下响应略强，这为探针1在生理条件下的应用提供了可能。
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图 7 pH对探针性能的影响 Fig.7 pH dependence of fluorescence intensities λex = 370 nm, λem = 522 nm, Slit = 5 nm/5 nm |
用滤纸实验(如图 8所示)验证了探针1的便捷性。将滤纸条浸没到探针1的乙腈溶液中，取出滤纸条，待溶剂蒸发后再将滤纸条浸没到PPh3的溶液中，将滤纸条置于手提式荧光灯下，可以明显观察到滤纸条从无色变为绿色(图 8b)，甚至在3个月后，荧光信号仍然可以明显观察到。由此可见，在溶液状态下和滤纸上，探针1均可以实现方便、灵敏地检测低浓度的PPh3。
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图 8 探针1在溶液中(左)和滤纸上(右)检测PPh3的视觉效果 (a)散漫日光下目视；(b)用手提式紫外灯照射(365 nm) Fig.8 The view of the detection of PPh3 with probe 1 in solution(left) and on filter paper(right) (a) under room light; (b) irradiated with a hand-held UV lamp (365 nm) |
本文设计合成了一个简单易得、高灵敏性和专一性检测PPh3的荧光探针，为PPh3的检测提供了一种方便的分析方法。该荧光增强型探针1借助Staudinger连接反应类似的机制，与PPh3反应后所掩蔽的荧光团释放出荧光。线性范围在2 ~10 μmol/L，最小检测限低于1 μmol/L，在较宽的pH值范围内均有较好响应。该探针不仅能在乙腈水溶液中检测PPh3，也可以便捷地在滤纸上实现其定性检测，具有潜在应用价值。
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