冷　玲,李　壹,熊曉輝

(南京工業(yè)大學食品與輕工學院,江蘇南京 211816)

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重金屬檢測熒光傳感技術(shù)的研究進展

冷玲,李壹,熊曉輝*

(南京工業(yè)大學食品與輕工學院,江蘇南京 211816)

重金屬離子是一類極具生理毒性的化學物質(zhì),其檢測方法在化學傳感領(lǐng)域引起了人們廣泛的關(guān)注,而食品中重金屬離子的檢測也愈發(fā)重要。熒光探針因具有組織穿透性、低背景熒光干擾、高效靈敏和檢測實時便捷等特點,成為重金屬檢測的重要手段之一。文中綜述了近年來主要重金屬離子檢測熒光探針(香豆素類,羅丹明類、喹啉類和比率型)的研究進展,重點分析熒光探針的設(shè)計原理,檢測機制及其結(jié)構(gòu)與化學傳感的關(guān)系,為熒光探針識別重金屬的應(yīng)用提供指導。最后展望了熒光探針在食品檢測、環(huán)境監(jiān)測和生物成像等領(lǐng)域的發(fā)展趨勢和應(yīng)用前景。

熒光探針,重金屬,傳感器,食品檢測

隨著人們生活水平的日益提高,食品安全問題也成為了百姓關(guān)心的頭等大事。近年來,“毒大米”、“小龍蝦中毒”等事件的頻發(fā),嚴重危害身體健康,讓人們對重金屬離子的檢測愈發(fā)重視。

重金屬廣泛地存在于自然界中,主要指汞(Hg)、銅(Cu)和鉛(Pb)等元素。即使在很低的濃度下,汞也具有很高的毒性[1],其主要通過消化道,呼吸道和皮膚吸收到體內(nèi),對神經(jīng)和內(nèi)分泌造成實質(zhì)性損害[2],汞離子對蛋白質(zhì)中的巰基有高親和力[3],造成大腦,腎和中樞神經(jīng)系統(tǒng)的諸多健康問題[4],如甲狀腺腫大和水俁病等。銅離子是一個顯著的金屬污染物,美國環(huán)境保護署(EPA)規(guī)定飲用水中的銅離子含量上限是1.3 ppm[5]。研究表明銅離子的細胞毒性可導致嚴重疾病,如印第安人兒童肝硬化(ICC)[6],朊病毒病[7],肌萎縮性側(cè)硬化癥[8]和威爾遜疾病[9]。鉛離子是一種對人體有害的神經(jīng)毒素,通過食物鏈進入機體,在體內(nèi)富集蓄積造成急慢性的中毒,對機體新陳代謝產(chǎn)生較大的干擾[10]。

由于重金屬污染的擴散以及對環(huán)境造成污染的嚴重性,開發(fā)高靈敏度、高精密度的儀器檢測方法是必要的。本文概述了傳統(tǒng)重金屬檢測技術(shù)的研究與發(fā)展,重點闡述了重金屬熒光傳感快速檢測技術(shù),旨在推動重金屬熒光傳感技術(shù)的發(fā)展,進一步完善食品安全檢測技術(shù)體系。

1　重金屬檢測技術(shù)的研究與發(fā)展

圖1　基于香豆素的探針[20-22]Fig.1　Chemosensors based on coumarin[20-22]

當前,常見傳統(tǒng)檢測重金屬的方法有:電化學分析[11],原子吸收光譜(AAS)[12],電感耦合等離子體質(zhì)譜(ICP-MS)[13],電感耦合等離子體原子發(fā)射光譜法(ICP-AES)[14],伏安法[15],比色法[16],紫外光譜[17]。上述方法大都為檢測重金屬的傳統(tǒng)方法,方法靈敏度高,但是也存在操作繁瑣、樣品前處理時間長、檢測時間長和需要大型儀器等諸多局限,因此不能實現(xiàn)樣品快速檢測。為實現(xiàn)快速、有效準確的現(xiàn)場檢測,出現(xiàn)了酶分析法、免疫分析法和目視比色法等重金屬快速檢測方法,其具有檢測快速、操作簡便、成本低廉的優(yōu)點。重金屬的快速檢測方法正好與傳統(tǒng)的儀器檢測方法相互補,在重金屬檢測的常規(guī)化、現(xiàn)場檢測、即時篩選等方面具有獨特的優(yōu)勢。雖然當前多數(shù)快速檢測方法對環(huán)境污染物的檢測還只能達到定性(或半定量)檢測的程度,而且檢測的靈敏度和準確性也不如傳統(tǒng)的儀器檢測方法。

分子熒光探針技術(shù)也是一種重金屬快速檢測技術(shù),已成為傳感和成像的有力工具,廣泛應(yīng)用于環(huán)境和生物樣品中,而在食品領(lǐng)域鮮有研究[18]。其中,有機染料香豆素、羅丹明、喹啉及其比率型為代表的熒光分子探針,在飲用水、茶葉和魚類等各種實際食品樣品重金屬快速檢測和現(xiàn)場分析方面有著重要應(yīng)用,為我國食品安全做出貢獻。

2　熒光探針對重金屬離子的檢測

熒光探針一般由熒光信號報告單元(reportor)和底物接收單元(receptor)兩個主要基本要素構(gòu)成。底物與探針受體單元通過各種作用力(如分子間氫鍵,靜電作用,共價鍵等)相互作用,引起信號報告單元熒光信號的改變,進而實現(xiàn)對目標物的傳感檢測。其信號傳遞機制包括:光致電子轉(zhuǎn)移(PET)、熒光共振能量轉(zhuǎn)移(FRET)和分子內(nèi)電荷轉(zhuǎn)移(ICT)等。常見的熒光探針大致可分為:香豆素類,羅丹明類,喹啉類和比率型等。

2.1香豆素類

香豆素具有相對光穩(wěn)定性,低毒性,易修飾和較長的激發(fā)波長。香豆素及其衍生物具有很大的斯托克斯位移,一定可調(diào)性的光物理參數(shù)和優(yōu)良的量子產(chǎn)率,進而有效地防止了激發(fā)和發(fā)射光譜之間的重疊[19]。Xu等[20]設(shè)計合成了一種基于香豆素的新型汞離子熒光探針1,即一種氨基硫脲衍生物,該探針可檢測哺乳動物細胞或大腸桿菌中的Hg2+。Hg2+誘導氨基硫脲衍生物的脫硫反應(yīng),在495 nm處有明顯的綠色熒光增強,最大發(fā)射波長有10 nm的紅移表明發(fā)生了熒光分子內(nèi)電荷轉(zhuǎn)移(ICT)。該新型熒光探針在乙醇-水溶液體系中表現(xiàn)出良好的選擇性和實用性,具有更廣的應(yīng)用和生物學意義。Yeh等[21]報道了一種含苯酚腙基團的香豆素類熒光探針2,當加入Cu2+時,體系出現(xiàn)從淡黃色到紅色的顏色變化,向傳感系統(tǒng)中加入CN-,發(fā)射峰強度顯著增大;再次加入Cu2+,熒光探針2的熒光再次猝滅,證實了探針2和Cu2+之間的結(jié)合是可逆的。即使存在其他金屬離子的干擾,Cu2+也是造成顯著熒光猝滅的唯一金屬;Cu2+處理Hela活細胞后,細胞內(nèi)部的強熒光淬滅,實現(xiàn)了活細胞中Cu2+的識別。Yu等[22]設(shè)計的熒光探針3是含亞胺和羥基的香豆素類衍生物,因Hg2+抑制了亞胺的異構(gòu)化,導致探針3藍色熒光大大增強。熒光顯微鏡實驗證實該探針能用于活細胞內(nèi)檢測Hg2+,其他重金屬離子對檢測無顯著干擾。圖1是基于香豆素的探針。

2.2羅丹明類

圖2　基于羅丹明衍生物的探針[26-28]Fig.2　Chemosensors based on rhodamine analogue[26-28]

2.3喹啉類

Sarkar等[29]將8-氨基喹啉和3,5-二氯-2-羥基苯甲醛以1∶1摩爾比的席夫堿縮合,合成了一種基于喹啉的可逆探針10,可高效率檢測生理和環(huán)境樣品中的Zn2+,其檢出限為5.0×10-9mol/L。由于激發(fā)態(tài)分子內(nèi)質(zhì)子轉(zhuǎn)移(ESIPT),傳感器具有弱熒光強度;存在Zn2+時,激發(fā)態(tài)分子內(nèi)質(zhì)子轉(zhuǎn)移過程受到配位作用的抑制,導致該探針在553 nm處的發(fā)射強度提高53倍;良好螯合劑EDTA使探針脫離分析物,造成熒光猝滅;再次加入Zn2+,探針可再次使用。EDTA的加入反映了探針的一種強選擇性的“關(guān)-開-關(guān)”熒光信號變化。密度泛函理論(DET)和含時密度泛函理論(TDDFT)計算出了探針的電子結(jié)構(gòu)和傳感機制。晶體結(jié)構(gòu)顯示Zn2+中心的周圍以N,N,O-三齒單陰離子形式結(jié)合了兩個有偽八面體幾何的受體分子(探針)。Basa等[30]探討了一種少見的多離子反應(yīng)和單分子系統(tǒng)的化學傳感器和化學計量器11,即蒽酮-喹啉亞胺衍生物的選擇性還原,在95%乙醇中使用1.0當量的NaBH4得到相應(yīng)的蒽-9-醇衍生物。Zn2+的配位作用誘導1,5-質(zhì)子化轉(zhuǎn)移,使喹啉取代蒽酮-亞胺分子,導致延伸的π共軛蒽熒光團通過亞胺-烯胺互變異構(gòu)途徑形成。當加入5.0當量的Zn2+,該傳感器的藍色發(fā)射熒光增強了42倍,吸收光譜有一個紅移。Cu2+誘導傳感器出現(xiàn)了從淺黃色到橙紅色的比色變化,傳感器發(fā)生不可逆的亞胺水解,熒光完全猝滅。Li等[31]以喹啉基團作為熒光單元,吡啶-2-甲胺作為金屬離子的結(jié)合單元,合成了一個簡單的熒光探針12。X射線晶體結(jié)構(gòu)分析表明該傳感器與Zn2+以1∶1的比例進行配位,而和Cd2+以2∶1的比例進行結(jié)合,這導致這些絡(luò)合物的熒光單元有不同的空間排布。加入Zn2+,該傳感器在316 nm處的吸收峰逐漸下降,而在363 nm處出現(xiàn)一個新的吸收峰,表明絡(luò)合物中的Cd2+能被Zn2+取代。乙腈溶液中,用Zn2+滴定傳感器,會使498 nm處出現(xiàn)熒光發(fā)射并有75倍的熒光增強。HOMO和傳感器的LUMO上的π-電子主要集中在喹啉上,當加入Cd2+時,LUMO和HOMO上的能級高于傳感器;存在Zn2+時,HOMO和LUMO上的π-電子分布有一個明顯的變化。通過與Zn2+結(jié)合,電子轉(zhuǎn)移過程大大抑制LUMO的穩(wěn)定,相比Cd2+,導致更大熒光增強。當用Zn2+處理MCF-7細胞時,明場成像中有綠色熒光,表明傳感器的生物相容性適用于快速檢測活細胞內(nèi)的Zn2+。圖3表示基于喹啉的探針。

圖3　基于喹啉的探針[29-31]Fig.3　Chemosensors based on quinoline[29-31]

2.4比率型

圖4　比率型探針[40-43]Fig.4　Radiometric chemosensors.[40-43]

通常熒光傳感器分為基于強度的和比率傳感器。在實際分析中,熒光探針的熒光強度通常受很多因素影響[32-33],如光照強度,光路長度,探針濃度,激發(fā)強度和檢測環(huán)境(pH,極性,溫度等)[34-35]。因此,基于強度的傳感器是在熒光強度的單變量測量易受定量檢測的干擾[36-37]。原則上,比率傳感器通過測量在兩個不同波長下的熒光強度比率[38],提供一個環(huán)境因素的內(nèi)置校正[39]。圖4是四種比率型探針。Zhang等[40]合成了一種新型香豆素和喹啉的雙熒光基團比率Cu2+探針13?？瞻讉鞲衅髟?22 nm處有最大吸收,當加入Cu2+時,由于喹啉環(huán)和吡啶基之間的共面效應(yīng),增加了共軛面積和減少了HOMO-LUMO之間的能隙,Cu2+絡(luò)合物導致最大吸收出現(xiàn)從322 nm到415 nm的紅移。當設(shè)定290 nm處激發(fā)時,傳感器溶液在355 nm處熒光強度沒有變化,而470 nm處有明顯熒光猝滅,這是因為光激發(fā)喹啉環(huán)到低能空d-軌道的能量轉(zhuǎn)移。355 nm處的發(fā)射光可作為穩(wěn)定的內(nèi)標,且在比率檢測中,熒光信號的雙變到單變量的轉(zhuǎn)化。470、355 nm處的發(fā)射率(I470 nm/I355 nm)線性下降,說明Cu2+濃度不斷增加。Gong等[41]設(shè)計合成了一種香豆素-羅丹明TBET系統(tǒng)的比率探針14。通過采用Hg2+脫硫反應(yīng)作為識別機理,呈現(xiàn)對Hg2+的強選擇性和高靈敏性?？瞻滋结?[Hg2+]=0 mol/L在420 nm處有明顯的吸收峰,伴隨供體(香豆素)發(fā)出的微黃色;且無受體的吸收峰特性(羅丹明B,近560 nm),證明了螺內(nèi)結(jié)構(gòu)的存在。但是,當Hg2+加入緩沖溶液后,供體的吸收條帶出現(xiàn)紅移。這種結(jié)果可能是因為Hg2+-觸發(fā)氨基硫脲向1,3,4-惡二唑轉(zhuǎn)化,增加了共軛的程度,表明對香豆素有較強的吸電子作用。除了香豆素吸收峰的紅移,在567 nm處出現(xiàn)了一個新的對應(yīng)羅丹明B開環(huán)結(jié)構(gòu)的特征吸收峰,具有很高的摩爾消光系數(shù),隨著Hg2+濃度增加,吸收強度增加。同時,肉眼很容易觀察到,傳感器顏色從微黃色到紅色的顯著變化,熒光從青色到粉紅色的變化(365 nm處激發(fā))。Sun等[42]報道了一種萘胺-羅丹明比率型和比色型探針15。探針能通過熒光成像用于監(jiān)測活鼠中Pd2+。在金屬離子中,探針對Pd2+有很強地選擇性。無Pd2+時,乙醇/水(1∶1)中的探針(10 μmol/L)在490 nm處有最大發(fā)射峰。無論是固態(tài)還是溶液中,探針發(fā)射出藍綠色的熒光(萘胺部分的特性),紅色熒光消失表明無金屬離子的溶液中,羅丹明部分的螺內(nèi)環(huán)關(guān)閉。加入Pd2+離子后,熒光強度比率(I590nm/I490nm)前5 min內(nèi)迅速增加,10 min內(nèi)達到最大值。不斷加入Pd2+,490 nm處的熒光強度逐漸降低,而590 nm處出現(xiàn)一個新的最大發(fā)射峰,且逐漸增強;同時,萘胺的藍綠色熒光猝滅,羅丹明的螺內(nèi)環(huán)打開,伴隨紅色熒光。向上述溶液中滴加EDTA-2Na,590 nm處的熒光強度逐漸降低,而490 nm處的最大峰強度增加,過量的EDTA-2Na使590 nm處的熒光完全猝滅。這歸因于探針上Pd2+被除去,羅丹明部分的螺內(nèi)環(huán)重構(gòu)。Zhou等[43]設(shè)計了一個含羅丹明和氨基喹啉基團的簡單熒光探針16。根據(jù)Job plot 實驗,探針和Hg2+以1∶1化學計量連接。Hg2+逐漸加入探針溶液中,探針(5×10-5mol/L)在497 nm處的發(fā)射峰逐漸下降,伴隨581 nm處一個新的峰形成,且581 nm處的熒光強度隨著Hg2+濃度增加而逐漸增加,之后達到平衡,伴隨溶液由無色變?yōu)榧t色,發(fā)出亮綠色熒光。在420 nm處,激發(fā)絡(luò)合物喹啉-鋅部分,探針可能發(fā)生熒光能量共振轉(zhuǎn)移(FRET)現(xiàn)象。此外,熒光光譜的比率變化在547 nm處分離發(fā)射點。

3　總結(jié)和展望

綜上所述,近年來,熒光探針用于檢測重金屬的研究快速發(fā)展,新型材料[44-47]的設(shè)計合成使得重金屬檢測在食品檢測、環(huán)境監(jiān)測和生物毒理學等領(lǐng)域不斷拓寬。實際樣品的基質(zhì)復雜,熒光探針易受多種檢測環(huán)境干擾,其檢測準確性和利用率較低,因此突破傳統(tǒng)的設(shè)計理念,構(gòu)建多功能和高性能檢測體系,開發(fā)新的檢測機理是一項重要的研究內(nèi)容。同時保持傳感器優(yōu)良的檢測性能,開發(fā)可重復利用,環(huán)境友好和實時監(jiān)測的傳感器,也是目前和未來的一個研究熱點?？傊?多種技術(shù)之間交叉融合,取長補短,新型化學傳感器的設(shè)計合成將指日可待。

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Study on fluorescent chemosensors for heavy metal ions

LENG Ling,LI Yi,XIONG Xiao-hui*

(College of Food Science and Light Industry,Nanjing Tech University,Nanjing 211816,China)

As a great toxic chemical substance in physiology,heavy metal ions in food causes severe healthy problems and the detection methods of them have aroused widespread concern,especially in sensing area. Owing to their unique advantages,such as tissue penetration,minimum interference from background auto-fluorescence and efficient detection,fluorescent chemosensors have been one of the most important sensing methods for heavy metals. The advances in the progress of fluorescent chemosensors including coumarin,rhodamine analogue,quinoline and radiometric chemosensors were reviewed in this paper. The design principles of chemosensors,as well as sensing mechanisms and the relationship between structure and chemical sensing were analyzed,which provided guidance for fluorescent chemosensors to identify the heavy metals. Finally,in the aspect of food testing,environmental monitoring and biological imaging,the development and prospect of fluorescent chemosensors were addressed to our understanding.

fluorescent probes;heavy metal ions;chemosensors;food detection

2015-12-16

冷玲(1992-)，女，碩士研究生，研究方向：食品安全，E-mail：learning92@163.com。

熊曉輝(1964-)，男，博士，教授，研究方向：食品安全，E-mail：xxh@njtech.edu.cn。

“十二五”農(nóng)村領(lǐng)域國家科技計劃(2013BAD19B09)；江蘇省科技基礎(chǔ)設(shè)施建設(shè)計劃( BM2012026)。

TS201.6

A

1002-0306(2016)15-0380-06

10.13386/j.issn1002-0306.2016.15.066