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锰掺杂硫化锌量子点室温磷光检测镉离子

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赵 磊,苗艳明,李瑜婷,连林旺,闫桂琴

(山西师范大学生命科学学院,山西 临汾 041004)

摘要:作为一类理想的磷光探针,量子点近年来在环境污染物定性定量分析方面应用广泛。量子点是半径小于或接近于激子玻尔半径的一类半导体纳米晶。以巯基丙酸(MPA)为表面修饰剂制备了稳定的水溶性掺杂型ZnS∶Mn2+量子点并应用于金属Cd2+的检测。在pH 7.0的PBS缓冲介质中,Cd2+可使ZnS∶Mn2+体系磷光猝灭,强度变化与Cd2+浓度呈良好线性关系,其线性范围为8×10-8~8×10-6 mol/L,方法检测下限为3.86×10-8 mol/L;利用荧光光谱、紫外可见吸收光谱研究了ZnS:Mn2+纳米晶结构及其光谱特性,探讨了识别Cd2+的可能机理。该方法应用于汾河水中Cd2+的检测,回收率为93.2%~97.1%。

教育期刊网 http://www.jyqkw.com
关键词 :量子点;室温磷光检测(RTP);镉离子

中图分类号:O657.3 文献标识码:A 文章编号:0439-8114(2015)16-4033-05

DOI:10.14088/j.cnki.issn0439-8114.2015.16.052

收稿日期:2014-9-10

基金项目:山西省化学优势重点学科建设项目(912019)

作者简介:赵 磊(1989-),男,山西太原人,在读硕士研究生,研究方向为生态化学,(电话)15835780799(电子信箱)shitou890410@126.com;

通信作者,闫桂琴(1956-),教授,博士,主要从事植物分子生态学研究,(电话)13513652319(电子信箱)gqyan@126.com。

量子点(Quantum dots,QDs) 即半径小于或接近于激子玻尔半径的半导体纳米晶粒,是一种零维的纳米材料,尺寸在纳米级的金属或半导体材料的细小颗粒,尺寸范围为1~100 nm。量子点具有许多块体材料和分子级别材料所不具备的性质,如:量子尺寸效应、表面效应、宏观量子隧道效应和介电限域效应等,并由此派生出量子点独特的发光特性。与传统的有机染料相比,量子点具有宽而连续的激发光谱、窄而对称的发射光谱、可精确调谐的发射波长(通过控制粒径来调整发射波长)、可忽略的光漂白等优良特性,使得其作为一种理想的磷光探针,在生物标记、成像及检测中应用广泛[1-8],目前将量子点用于检测离子[9-11],生物大分子[12-15]与小分子[16-18]正成为研究热点。室温磷光法较之荧光分析法,磷光寿命比荧光长,可避免自体荧光和散射光的干扰,且磷光的选择性优于荧光[19-25]。因此,可采用量子点的磷光特性开展检测技术研究[26]。

镉是一种广泛分布于环境中的重金属元素[27],采矿、冶炼、化石燃料等都会导致环境中的镉积累,并进入人类食物链[28],导致肾功能不全,钙代谢异常以及引发癌症[29]。而加强检测环境样品,工业废物排放和组织样本中的镉含量,将有利于控制人类镉的暴露水平。目前检测Cd2+的主要方法有原子光谱法[30]、电化学方法[31]、毛细管电泳法[32]、电感耦合等离子体质谱法[33]、分光光度法和荧光光谱法[34]等。本试验通过制备水溶性掺杂型ZnS∶Mn2+量子点,初步分析了对Cd2+的检测参数,以期为开发相关快速检测方法提供参考

1 材料与方法

1.1 材料和试剂

巯基丙酸(SPA)(北京百灵威科技有限公司),Zn(CH3COO)2·2H2O,Mn(CH3COO)2·4H2O,Cd(NO3)2,Na2S·9H2O(天津市科密欧化学试剂有限公司)均为分析纯,去离子水。

1.2 主要仪器

JSM-7500F透射电镜(JEOL,日本),UV-29100型分光光度计(日立,日本)紫外-可见分光光度计,Cary Eclipse荧光分光光度计(瓦里安, 美国),pH计(金鹏分析仪器有限公司)。

1.3 方法

1.3.1 Mn掺杂ZnS量子点的合成 取100 mL三口烧瓶,依次加入50 mL 0.04 mol/L巯基丙酸,5 mL 0.1 mol/L的Zn(CH3COO)2和2 mL 0.01 mol/L的Mn(CH3COO)2,混合后在室温下通氩气,用1 mol/L的NaOH调节pH至11后,搅拌30 min,然后快速注射0.1 mol/L的Na2S 5 mL,迅速搅拌20 min后,于50 ℃陈化2 h形成巯基丙酸包裹的Mn,最后通过与相同体积的乙醇沉淀进行离心纯化,在室温真空下干燥,得到高水溶性的量子点粉末[35],待用。

1.3.2 测量 在295 nm激发波长的磷光模式下,激发和发射狭缝宽度分别为10 nm和20 nm,在一系列10 mL比色管中,依次加入500 μL 0.02 mol/L的PBS缓冲液(pH=7.0),50 μL 2 mg/mL的上述量子点溶液,然后加入相同浓度不同体积的Cd2+水溶液,并以去离子水定容至5 mL,静置5 min后测定3次。

2 结果与分析

2.1 量子点性质分析

制备的水溶性量子点结构式见图1(a),其透射电镜图(b)表明Mn掺杂ZnS量子点具有球形形状,直径约为3.5 nm。其磷光激发和发射峰位于590 nm处。ZnS量子点只有缺陷态发光,而Mn掺杂ZnS量子点会发射磷光,起源于Mn2+的4T1-6A1跃迁[21]。

2.2 Mn掺杂ZnS量子点的RTP分析

Cd2+对Mn掺杂ZnS量子点磷光的影响见图2,结果表明Cd2+对Mn掺杂ZnS量子点的磷光具有猝灭效应。随着Cd2+浓度增加,量子点的RTP强度呈下降趋势,表明该量子点可用于镉离子的RTP探针。在最佳条件下,磷光猝灭强度与镉离子浓度的标准曲线见图3。由图3计算其线性回归方程为ΔP=0.000 4 C+1.010 2,相关系数为0.993 5,连续测定11次不含镉离子和含有0.2 μmol/L镉离子磷光差值的相对标准偏差为1.8%。计算该方法的Cd2+检出限为3.86×10-8 mol/L。

2.3 RTP探针的性质探讨

为鉴定Cd2+在该分析体系中的特异性,分析了体系中的探针磷光特性,结果见图4。由图4可知,Mn掺杂ZnS量子点的磷光发射峰激发于595 nm,在Mn掺杂ZnS量子点体系中添加Cd2+,可显著降低体系磷光强度,且随着Cd2+浓度增加,其荧光强度有规律地降低,即Mn掺杂ZnS量子点可与Cd2+发生相互作用。

紫外-可见光谱分析结果见图5。由图5可知,Mn掺杂ZnS量子点的光强度较低,但加入镉离子后,在量子点表面形成了S-Cd键,增大了整个体系的发光强度[36]。

磷光猝灭过程通常分为动态碎灭(遵从Stem-Voliner方程Eq.1)和静态碎灭(遵从Lineweaver- Burk方程Eq.2)两类[37,38]:

其中,P0代表磷光体磷光强度,P代表加入磷光碎灭剂后体系的磷光强度,cq为碎灭剂Cd2+浓度,Ksv是动态猝灭常数,KLB是静态猝灭常数[39-41]。P0/P和cq的关系不遵循Stern-Volmer方程,而(P0-P)-1与cq的关系符合Lineweaver-Burk方程,说明Cd2+猝灭Mn掺杂ZnS量子点是一个静态猝灭过程(图6),即二者相互作用后产生了非磷光物质。

2.4 检测体系的优化

为优化检测体系,试验研究了pH、反应时间以及NaCl浓度对40 mg/L Mn掺杂ZnS量子点RTP强度的影响。由图7可知,当pH为4.5~9.5时,随着pH增加,量子点RTP强度呈先增后减,并在pH 6~8.5时趋于稳定,考虑普通环境水样pH在7左右,故选择反应体系pH为7。由图8和图9可知量子点在60 min内和高NaCl浓度下,RTP强度基本稳定。

2.5 样品分析

取一定量汾河水,过滤后,采用加标回收法分析,结果见表1。由表1可知,样品回收率达到93%以上,检测相对标准偏差小于6%,初步符合检测分析要求。

3 结论

采用MPA包裹的Mn掺杂ZnS量子点可为快速检测镉离子提供新思路,该法不需复杂的样品预处理,操作简单,且采用的磷光检测体系,可有效避免生物体液的自体荧光和散射光干扰,勿需除氧剂和诱导剂,成本低,是一种简单、快速、经济、灵敏和高选择性的检测水样中镉离子的方法。

教育期刊网 http://www.jyqkw.com
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