Plasmonic Metallic Nanostructures as Colorimetric Probes for Environmental Pollutants

CHAPTER

PLASMONIC METALLIC NANOSTRUCTURES AS COLORIMETRIC PROBES FOR ENVIRONMENTAL POLLUTANTS

11

Jingbin Zeng, Wei Duan, Mengmeng Li, Yitong Xue College of Science, China University of Petroleum (East China), Qingdao, China

INTRODUCTION Colorimetric assay is a powerful tool for the detection targets due to its quick feedback, visual readout, and simple operation. Typically, molecular chromophores with delocalized conjugated functional groups are employed as colorimetric receptors, but they often suffer from low sensitivity and the lack of ability to be functionalized for complicated sensing environment. Inspired by their high extinction coefficients and distance-dependent optical properties, plasmonic metallic nanostructures such as gold, silver, and their composite have recently emerged as fascinating optical probes, which have found numerous applications in the colorimetric detection of heavy metals [1,2], chemical ware agents [3], toxic anions [4], etc.

LOCALIZED SURFACE PLASMONIC RESONANCE Localized surface plasmonic resonance (LSPR) is a phenomenon in which the conducting electrons at the surface of metals are collectively oscillated when their frequency matches that of the incident electromagnetic radiation. As a consequence of this phenomenon, plasmonic metal nanoparticles (PMNPs) (e.g., gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs)) show strong absorption within certain wavelength located in (near) visible region and therefore have corresponding colors. The LSPR is intimately related to PMNP size, composition, morphology, interparticle distances, and refractive index of the surrounding media, thereby enabling designs of signal transducers for visual recognition [5]. These LSPR-based detection strategies can be divided into four categories including targetstimulated aggregations, antiaggregations, etching, and formation of new plasmonic nanostructures.

Novel Nanomaterials for Biomedical, Environmental and Energy Applications. https://doi.org/10.1016/B978-0-12-814497-8.00011-4 Copyright # 2019 Elsevier Inc. All rights reserved.

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TARGET-TRIGGERED AGGREGATION OF METALLIC NANOSTRUCTURES The most prevalent strategy of using LSPR for colorimetric sensing is based on the target-induced aggregation of PMNPs that will lead to coupling of their electromagnetic fields and thus redshifts of absorption bands dependent on interparticle distance. These redshifts of LSPR are along with color that changes from red to blue (AuNPs) or yellow to brown (AgNPs), which can be utilized for the naked-eye detection of analytes. To achieve this goal, PMNPs are always modified with organic or biological ligands, which are able to coordinate with analytes of interest and thus trigger the aggregation of metallic nanoparticles into larger assemblies [6]. Due to limited space, we will introduce only several typical and recent examples using this approach for the colorimetric sensing of target analytes.

DETECTION OF METAL IONS Mirkin’s group developed an oligonucleotide AuNP probe for detecting Hg2+ [7]. In their method, two types of AuNPs were individually functionalized with two different DNA sequences that were complementary to each other except for a single thymidine-thymidine (T-T) mismatch. These functionalized AuNPs were assembled exclusively in the presence of Hg2+ since the two thymidine moieties can form a tightly bound complex with Hg2+. As the temperature was increased to the melting temperature, Tm, the double-stranded DNA dehybridized, leading to the dissociation of aggregated AuNPs along with a reversible color change from blue to red. None of the common metal ions other than Hg2+ could raise the Tm obviously (c.5°C) via the formation of T-Hg2+-T complex as it is known that the Tm is increased in the presence of DNA duplex. This melting transition property improved the sensitivity and lowered the limit of detection (LOD) visibly to 100 nM (20 ppb). Zhou et al., for the first time, introduced click chemistry for the AuNP-based colorimetric detection [8]. They synthesized azide- and terminal alkyne-functionalized thiols and modified them onto AuNPs through Au-S bond. Cu(II) was first reduced to Cu(I) that was then able to catalyze the cycloaddition reaction between azides and alkynes in click chemistry based on Huisgen’s reaction and led to the aggregation of modified AuNPs. This assay allows for highly specific detection of Cu(II) over all the other cations tested. The same research group further introduced a highly sensitive and selective method to detect Hg2+ by AuNPs modified with quaternary ammonium group-terminated thiols. As shown in Fig. 11.1, the modified AuNPs were positively charged and remained dispersed in the solution, which, instead, tended to aggregate as long as the thiols were extracted by Hg2+. This assay was highly selective in view of the fact that only Hg2+ is capable of removing thiolates chemisorbed on Au surface [9]. Another well-known strategy was proposed based on the metal-ligand coordination chemistry where the metal is an electronic acceptor and the ligand is the electronic donor. Alizadeh et al. prepared octanethiol-capped AuNPs and separately modified them with two pyridine moieties, that is, chelidamate and 2-aminopyridine. The addition of Cu2+ and Ag+ induced interparticle cross-linking of modified nanoparticles through interfacial 2 + 1 complexation with chelidamic acid (chelidamic acid/Cu2+/ chelidamic acid) or 2-aminopyridine (2-aminopyridine/Ag+/2-aminopyridine) moieties [10]. Similarly, di-(1H-pyrrol-2-yl)methanethione (DP) was synthesized and used to functionalize AuNPs through the Au-S bond [11]. Among all the tested ions, Cd2+ was the only ion that chelated selectively with the dipyrrole moiety on DP-AuNPs and caused their aggregation. A linear relationship was found between the absorbance and the Cd2+ concentration from 0.5 to 16 μM. This method was applied to the

TARGET-TRIGGERED AGGREGATION OF METALLIC NANOSTRUCTURES

329

N+

HS

Au

Au Hg2+ ( )

Au

Blank AI3+ Ca2+ Cd2+ Cr3+ Cu2+ Fe3+ Hg2+ Mn2+ Pb2+

FIG. 11.1 AuNPs modified with quaternary ammonium for the colorimetric detection of Hg2+. Reproduced with permission from Liu D, Qu W, Chen W, Zhang W, Wang Z, Jiang X. Highly sensitive, colorimetric detection of mercury(II) in aqueous media by quaternary ammonium group-capped gold nanoparticles at room temperature. Anal Chem 2010;82:9606–10. Copyright 2010, American Chemical Society.

HS SO3Na

HS

+

N

Br −

M3+ Ligand Exchange

Centrifugation OH−

FIG. 11.2 Proposed mechanism of the M3+-induced aggregation of AuNPs modified with oppositely charged thiols. Reproduced with permission from Zheng W, Li H, Chen W, Ji J, Jiang X. Recyclable colorimetric detection of trivalent cations in aqueous media using zwitterionic gold nanoparticles. Anal Chem. 2016;88:4140–6. Copyright 2016, American Chemical Society.

determination of Cd2+ in lake water, and the results obtained are in good agreement with those obtained using ICP-MS. Tan et al. provided a colorimetric method for the detection of As3+ using AuNPs modified with phosphonium ionic liquid, tetradecyl(trihexyl)phosphonium chloride ([C14(C6)3P]Cl) (P-IL). + At pH 10.2, As3+ mainly existed as H2AsO
3 , which had a strong interaction with C14(C6)3P through anion exchange. This method can be used for the determination of the total amount of inorganic As by prereducing As6+ into As3+ with ascorbic acid [12]. Recently, oppositely charged thiols were modified onto AuNPs to form intermolecular zwitterionic surfaces (Fig. 11.2). These zwitterionic AuNPs (zwAuNPs) were well dispersed and stable in high-salt solution in a wide pH range. However, in the

330

CHAPTER 11 PLASMONIC METALLIC NANOSTRUCTURES

presence of trivalent metal cations (M3+), the surface potential of zw-AuNPs was significantly interfered resulting in NPs’ aggregation. This assay was highly valency-specific since it is exclusively responsive toward M3+, and more importantly, the zw-AuNPs can be regenerated and recycled by removing M3+ [13]. Some other studies utilized similar metal-ligand coordination chemistry for the detection of metal ions that are listed in Table 11.1. As introduced above, most of the aggregation-based strategies are mainly using AuNPs, while there are also some reports dealing with AgNPs taking advantage of their higher molar extinction coefficients (c.1 order higher than that of AuNPs) and lower cost. Bifunctionalization of AgNPs with 6-mercaptonicotinic acid (MNA) and melamine (MA) was developed to probe Cr3+ and Ba2+ simultaneously via the cooperative metal-ligand interaction [25]. The color of the solution changed from yellow, to reddish brown (Cr3+), to orange (Ba2+) with detection limit of 64.51 and 80.21 nM for Cr3+ and Ba2+, respectively. This method was successfully applied to detect Cr3+ and Ba2+ in samples of drinking water, tap water, and river water. Interestingly, the same group prepared 2-mercapto-5-nitrobenzimidazole (MNBZ)-capped AgNPs, which appeared to assemble in the presence of both Mg2+ and glyphosate through complex formation between MNBZ-AgNPs-Mg2+ and glyphosate [26]. The addition of Mg2+ did not cause aggregation of MNBZ-AgNPs, but the subsequent addition of glyphosate resulted in a drastic decrease in interparticle distance through complex

Table 11.1 Aggregation-Based Sensors or Metal Ions Via Metal-Ligand Coordination Metal Ions Ag

+

Method

Modified Ligand

Signal

Working Range

LOD

Refs.

AuNP-based aggregation

Peptide

Absorption at 700/550 nm Absorption at 700/520 nm Absorption at 625/530 nm Absorption at 650/520 nm Absorption at 750/530 nm Absorption at 650/520 nm Absorption at 570/540 nm Absorption at 735/400 nm Absorption at 522/396 nm Absorption at 635/390 nm Absorption at 510/390 nm

10–1000 nM

7.4 nM

[14]

0.03–2 μM

13 nM

[15]

20–120 μM

43.27 μM

[16]

1–100 μM

0.57 μM

[17]

1–9 nM

0.4 nM

[18]

0.6–1.4 μM

0.04 μM

[19]

2–50 ppm

35 ppm

[20]

–

5 nM

[21]

0.227–3.18 μM

0.13 μM

[22]

0–7.6 μM

0.36 μM

[23]

18.75–62.5 μM

6.25 μM

[24]

Pb2+

Glutathione

Ba2+ Al3+

11-Mercaptoundecylphosphonic acid 11-Mercaptoundecanoic acid

Cr6+

Citrate

Cr3+

Mecaptosuccinic acid

Ce3+

Biomolecules

Mn2+

AgNP-based aggregation

Cysteic acid

Cd2+

Chalcone carboxylic acid

Zn2+

Label-free

Cr3+

Cytosine triphosphate

TARGET-TRIGGERED AGGREGATION OF METALLIC NANOSTRUCTURES

331

formation between MNBZ-AgNPs-Mg2+ and glyphosate, yielding a color change from yellow to orange-red. The assay is applicable for the visual detection of Mg2+ or glyphosate. Buduru et al. developed a colorimetric method for the detection of Hg2+ using glutamine (Gln)- and histidine (His)functionalized AgNPs (Gln-His-AgNPs) as probes [27]. The recognition mechanism was attributed to unique supramolecular nanostructures of Gln-His-AgNPs, which yielded strong interaction (cooperative metal-ligand interaction) between Gln-His-AgNPs and Hg2+. A linear correlation was obtained in the range of 100–1000 μM with a detection limit of 25.48 μM. Other similar strategies based on metal-ligand interaction between metal ions and ligands on the AgNPs’ surface are also summarized in Table 11.1. Very recently, silver molecular nanoparticles (AgMNPs) were explored as optical probes [28]. As a proof of concept, M4Ag44(p-MBA)30 MNPs were synthesized as the optical probes for the detection of Cu2+. Stimulated by Cu2+, the AgMNPs gradually transformed into their larger counterparts, that is, AgNPs, and the size and ratio of the produced AgNPs increased with the increase of Cu2+ within a certain range of concentration. Concurrently, the color of the solution changed from wine red to purple, dark green, and finally brown. This method is also feasible for the spectrophotometric and Raman analysis of Cu2+.

DETECTION OF ANIONS Similar to some metal ions mentioned above, a number of studies pointed out that some anions also function as severe environmental pollutants and exhibit detrimental health effects. Anions either can be essential to sustained growth but should be controlled in a proper concentration range or serve as harmful contaminants. Ko et al. developed a dual-functional sensing system using AuNPs as probes for the analysis of CN
[29]. In this method, fluorescein isothiocyanate (FITC) molecules were chosen for the modification on the surface of AuNPs. The fluorescence was quenched by AuNPs due to the F€orster resonance energy transfer effect. CN
can react with gold in the presence of oxygen to form [Au(CN)
2 ] complex, thus removing FITC from the surface, which led to the recovery of fluorescence intensity. For the colorimetric strategy, polysorbate 20 (PS-20) was used to stabilize AuNPs against high ionic strength. Once the cyanide was added to the PS-20 functionalized AuNPs, the aggregation of AuNPs occurred due to the removal of PS-20 stabilizer by CN
. The color of the solution turned from red to blue-gray. Xiong et al. proposed a Janus PEGylated AuNP probe by separately functionalizing AuNPs with PEGs and recognition ligands (4-aminobenzenethiol (4-ABT)) [30]. As displayed in Fig. 11.3, with the existence of NO
2 , the AuNPs were linked together due to the generation of dimercaptoazobenzene formed by the reaction between 4-ABT and NO
2 , leading to the color change of the solution from red to purple. The proposed probes are able to function under extreme conditions such as high salts and broad pH. Kolekar’s group synthesized cetylpyridinium bromide-capped AuNPs (CPB-AuNPs) to recognize S2
over other coexisting substances in aqueous media [31]. The detection mechanism is based on the fact that the electrostatic interaction between S2
and positively charged AuNPs resulted in the aggregation of AuNPs. Later, they reported a similar method for the colorimetric detection of ClO
using CPB-AgNPs [32]. Wu et al. presented a method for the sensitive and selective colorimetric determination of I
. AuNPs were modified with triazole acetamide with the aid of Cu(I)-catalyzed click

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CHAPTER 11 PLASMONIC METALLIC NANOSTRUCTURES

FIG. 11.3 The NO
2 -induced generation of AuNP oligomers. Reproduced with permission from Xiong Y, Li M, Liu H, Xuan Z, Yang J, Liu D. Janus PEGylated gold nanoparticles: a robust colorimetric probe for sensing nitrite ions in complex samples. Nanoscale 2017;9:1811. Copyright 2017, the Royal Society of Chemistry.

reaction [33]. The presence of I
induced the aggregation of these AuNPs through the interaction with amide protons and resulted in a color change from wine red to purple. 4
Apyari’s group presented colorimetric methods for the detection of SO2
4 and P2O7 using positively charged AuNPs and AgNPs stabilized with 6,6-ionene [34,35]. With the addition of anions at 4
the concentration of 0.25 mg mL
1, only SO2
4 and P2O7 can drive the aggregation of NPs. The sorption of these anions on the NP’s surface decreased positive charge of NPs, leading to their cross-linking. They assumed that the different ability to form bonds with positively charged NPs was probably due to the different charge and size of these anions. They mentioned that the limits of detection were lower in the case of AgNPs compared with those of AuNPs. Boken et al. established a colorimetric method to recognize F
using thiobarbituric-capped AuNPs (TBA-AuNPs). The sensing mechanism is based on the fact that F
can form hydrogen bond with TBA and thus trigger the aggregation of AuNPs [36].

DETECTION OF SMALL ORGANIC MOLECULES Kanamycin is an aminoglycoside antibiotic broadly used for the treatment of many gram-negative bacterial infections. Lai et al. introduced a colorimetric assay for the quantitative determination of kanamycin [37]. The method utilized hydrogen-bonding recognition capability of AuNPs in a chitosan matrix. The hydrogen-bonding interaction between kanamycin and chitosan causes the aggregation of the AuNPs, which induced a color change of the AuNPs from wine red to blue. Kailasa’s group developed a colorimetric assay for the detection of pendimethalin herbicide using ractopamine-dithiocarbamate-capped AuNPs (RAC-DTC-AuNPs) as probes. [38] The pendimethalin can effectively induced the aggregation of RAC-DTC-AuNPs via different interactions such as donor acceptor, π-π, van der Waals, and hydrogen bonding (Fig. 11.4), giving rise to a color change from red to blue. Polycyclic aromatic hydrocarbons (PAHs) are known as persistent organic pollutants. The level of 1-hydroxypyrene (1-OHP) in urine had been proved to be a valuable marker for studying PAH exposure. Recently, a colorimetric approach for rapid determination of 1-OHP was reported by Li’s group, which relied on noncross-linking aggregation of AuNPs induced by 1-OHP with the coexistence of formic acid (FA) [39]. As shown in Fig. 11.5, the addition of FA into the AuNPs did not stimulate

O2N NO2 HO HN OH HO NO2

HO

N H NO2 HO

OH

HO

OH OH

S HO

S

S S

Au NPS

S N

S

S

N H

N

S

S

OH

HO

N

HO O2N

OH S S S N S N S Au NPS S HO S HO S S S N N HO NH

N H

OH

NO2 HO

RAC-DTC-Au NPs HO O2N

OH

OH O2N H N

S

NO2HO

O2N OH

NH

OH NO2

OH O2N

OH

N H O2N OH

HO

HO O2N HN

HN

NO2HO

OH NO2

OH O2N

O2N NO2 HO HN OH HO NO2

OH

N S

N

NO2 OH

S S S N S N S Au NPS S HO HO S S S N S N

OHO2N

S N S N S Au NPS S HO HO S S S S N N HO NH

OH

NO2 HO

OH NO2

O2N NO2 HO HN OH

Pendimethalin

OH

HO

O2N OH

NH

NO2HO

NO2

OH O2N H N

OH

N

HO NO2

HO

HO

O2N

OH

S

N HO

N

NOHO 2 H N

NH

N H NO2 HO

HO O2N

OH O2N H N

N OH S S S S N N S Au NPS S HO HO S S S N S N HO OH

NH NO2HO

O2N OH

NH

OH NO2

OHO2N

OHO2N

Pendimethalin induced aggregation of RAC-DTC-Au NPs

FIG. 11.4 A colorimetric assay for pendimethalin using RAC-DTC-AuNPs as probes. Reproduced with permission from Rohit JV, Kailasa SK. Simple and selective detection of pendimethalin herbicide in water and food samples based on the aggregation of ractopamine-dithiocarbamate functionalized gold nanoparticles. Sens Actuator B Chem 2017;245:541–50. Copyright 2017, Elsevier.

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O2N HO

333

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FIG. 11.5 Scheme of colorimetric sensing of 1-OHP. Reproduced with permission from Hu Y, Du C, Li Y, Fan L, Li X. A gold nanoparticle-based colorimetric probe for rapid detection of 1-hydroxypyrene in urine. Analyst 2015;140:4662–7. Copyright 2015, the Royal Society of Chemistry.

the aggregation of AuNPs, but a proton transfer was supposed to take place from FA to carboxylic anions on the surface of AuNPs decreasing the zeta potential. Following addition of 1-OHP into the FA-AuNP solution led to a further decrease of zeta potential and a hydrophobic environment, thereby enabling a strong and rapid noncross-linking aggregation of AuNPs with a color change from red to violet blue. Prothioconazole (PTC) is one of the triazole fungicides, which has been used for the control of fungal diseases on fruits, vegetables, and grain crops. Ivrigh et al. reported a sensitive colorimetric method based on the aggregation of citrate-capped AgNPs for the detection of PTC fungicide in wheat flour and paddy water [40]. Under suitable pH, the PTC molecule having multiamino groups was positively charged, and so, it would cross-link the negatively charged AgNPs due to electrostatic attraction between AgNPs and PTC molecules (Fig. 11.6). In addition, hydrogen bonding between the OH group of citrate ions and the OH or NH groups of PTC might also contribute to the aggregation of AgNPs. Endrin is one of the organochlorine pesticides having a cyclodiene group. Shrivas et al. proposed a colorimetric approach for the stereoselective detection of endrin pesticide in water and food samples using sucrose-capped AgNPs [41]. As shown in Fig. 11.7, the aggregation of AuNPs was initiated by the addition of endrin, which can displace sucrose via the high charge electron density of oxygen atoms to silver via nonbonded interactions. This assay is highly stereoselective since it shows neglectable recognition ability toward the isomeric molecule (dieldrin) possessing an oxygen at the exposition.

The citrate-capped AgNP-based assay for PTC. Reproduced with permission from Ivrigh ZJ, Fahimikashani N, Hormozinezhad MR. Aggregation-based colorimetric sensor for determination of prothioconazole fungicide using colloidal silver nanoparticles (AgNPs). Spectrochim Acta A Mol Biomol Spectrosc 2017;187:143. Copyright 2017, Elsevier.

TARGET-TRIGGERED AGGREGATION OF METALLIC NANOSTRUCTURES

FIG. 11.6

335

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CHAPTER 11 PLASMONIC METALLIC NANOSTRUCTURES

FIG. 11.7 (A) The sucrose-stabilized AgNPs and (B) aggregation of AgNPs in the presence of endrin. Reproduced with permission from Shrivas K, Nirmalkar N, Ghosale A, Thakur SS. Application of silver nanoparticles for a highly selective colorimetric assay of endrin in water and food samples based on stereoselective endo-recognition. RSC Adv 2016;6:2985562. Copyright 2016, the Royal Society of Chemistry.

TARGET-INDUCED ANTIAGGREGATION OF METALLIC NANOSTRUCTURES For the development of target-induced antiaggregation-based methods, a variety of organic compounds, which have multifunctional groups such as amino and thiol, were used. Since these functional molecules have strong affinity toward AuNPs or AgNPs, they are able to cross-link these NPs and induce color that changes from red to blue (AuNPs) or yellow to brown (AgNPs). However, in the presence of target analytes, the aggregation is either prevented or interfered since these analytes exhibit higher binding constants to these functional molecules than that of AuNPs or AgNPs. Thus, the color changed reversely from blue to red or brown to yellow dependent on the concentration of target analytes.

DETECTION OF METAL IONS An antiaggregation method for the detection of Pb2+ was developed using AuNPs bifunctionalized with carboxylate and 15-crown-5. In methanol/water system, the Pb2+ has a strong interaction with carboxylic and crown ether moieties giving rise to the break of hydrogen bond and the disaggregation of AuNPs. The color of the solution turned from blue to red, which is quantitatively related to the concentration of Pb2+ [42]. Liu and Lu modified AuNPs with Pb2+-specific DNAzyme that is composed of a substrate strand (17DS) and an enzyme strand (17E) as shown in Fig. 11.8 [43]. In order to assemble NPs, both ends of the substrate strand were extended by 12 bases that are complementary to the DNA attached on AuNPs. Without Pb2+, the AuNPs were assembled by the DNAzyme with a head-to-tail or tail-to-tail mode, while in the presence of Pb2+, substrate strand was cleaved by the enzyme strand, inhibiting the

substrate strand 17DS Pb(II)

3'

G T A G A G A A G G rA T A T C A C T C A 5'

5'

C A T C T C T T C T A T A G T G A G T 3' A A C C G G G T C enzyme strand17E G A C G C

(A)

(B) (II)

no

aggregate

= 17E =

(C)

50°C

Pb

eal ann Pb (II) an ne al

-S-(CH2)6-5'-CACGAGTTGACA-3' = DNAAu 5'-TGTCAACTCGTG ACTCACTAT rA = SubAu GGAAGAGATG TGTCAACTCGTG-3'

FIG. 11.8 Design of colorimetric sensor for Pb2+ by DNAzyme-directed assembly formation and cleavage of AuNPs. Reproduced with permission from Liu J, Lu Y. A colorimetric lead biosensor using DNAzyme-directed assembly of gold nanoparticles. J Am Chem Soc 2003;125:6642–3. Copyright 2003, American Chemical Society.

TARGET-INDUCED ANTIAGGREGATION OF METALLIC NANOSTRUCTURES

Cleavage site

337

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CHAPTER 11 PLASMONIC METALLIC NANOSTRUCTURES

assembly of NPs and giving rise to a red color. This assay is highly selective toward Pb2+ by taking advantage of the inherent specificity of DNA chemistry. Ye et al. introduced an antiaggregation method for the colorimetric detection of Cu2+ using PVPstabilized AuNPs and 2-mercaptobenzimidazole (MBI). In the absence of Cu2+, MBI can cause the aggregation of AuNPs by its imidazole moiety and thiol group. With the addition of Cu2+, the interaction between 2-mercaptobenzimidazole and AuNPs was blocked, for MBI has higher affinity toward Cu2+ than AuNPs [44]. Wang et al. reported an antiaggregation colorimetric approach for the detection of Hg2+ using unmodified AuNPs with the aid of thiourea (TU) [45]. TU has multifunctional groups like amino and carbon sulfide and is able to assemble AuNPs. However, the addition of Hg2+ can inhibit the aggregation of AuNPs since Hg2+ has stronger affinity toward TU than that of AuNPs preventing the aggregation of AuNPs stimulated by TU. The color changed from blue to purple and red, which was dependent on the concentration of Hg2+. This assay was highly selective owing to the fact that none of metal ions except Hg2+ exhibits better affinity toward TU than that of AuNPs. Likewise, cysteine has a thiol and amino group, and so, it can induce the aggregation of AuNPs via the formation of Au-S and Au-N bond. With the preincubation of cysteine with Hg2+, Hg2+-cysteine complexes were formed, and the concentration of free cysteine molecules was decreased, and therefore, the aggregation of AuNPs was interrupted [46]. AgNPs and Au nanorods (NRs) are also used along with various functional molecules for this purpose. To date, most of the AgNP-based colorimetric assays for the determination of Mn2+ have been developed based on Mn2+-triggered aggregation of ligand functionalized AgNPs. He et al., for the first time, proposed an antiaggregation method for the detection of Mn2+ using unmodified AgNPs and L-arginine with multiple amino groups [47]. The addition of L-arginine into AuNPs can trigger their cross-linking via the formation of AudN bond. Once L-arginine is premixed with Mn2+ before it’s mixed with AgNPs solution, the covalent interaction of L-arginine with AgNPs will be weakened due to the higher binding affinity of L-arginine toward Mn2+. Herein, a reverse process from assembly to dispersion took place, and the color of AgNPs was changed from colorless to yellow. Dithiothreitol (DTT), which has two thiol groups, can also lead to the assembly of AuNRs. Such assembly will be inhibited by the existence of Hg2+, which has higher binding affinity toward DDT than AuNRs does. Taking advantage of these properties, an antiaggregation method was developed for the colorimetric detection of Hg2+ [48]. Safavi’s group reported a colorimetric assay with excellent selectivity for detecting Ag+ in aqueous solution [49]. In their work, tris(hydroxymethyl)aminomethane (tris) was integrated as the cross-linking, complexing, and buffering agent. The detection mechanism of silver ions is illustrated in Fig. 11.9. The aggregation of AuNPs was induced by the electrostatic attraction between the positive head of tris and negative charges on AuNP-surface stemming from citrate ions. In addition, the hydrogen bonding between terminal hydroxyl groups of two neighboring tris molecules led to the decrease of the distance between AuNPs. In the presence of Ag+, the interaction between tris and AuNPs was interrupted by the competitive complexation reaction between Ag+ and tris, thereby impeding the agglomeration of AuNPs. As a consequence, upon increasing the Ag+ concentration, the color of AuNP-tris solution changed from dark blue to red.

DETECTION OF ANIONS Ye and coworkers modified AuNPs with 4-aminothiophenol (4-ATP), which can induce the agglomeration of AuNPs via the formation of Au-N and Au-S bonds (Fig. 11.10). When NO
2 exists at an acidic condition, it will react with amino groups in 4-ATP to form diazonium cation, reducing the number of

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339

FIG. 11.9 A scheme depicting the colorimetric assay for Ag+ using AuNPs and tris. Reproduced with permission from Safavi A, Ahmadi R, Mohammadpour Z. Colorimetric sensing of silver ion based on anti aggregation of gold nanoparticles. Sens Actuator B: Chem 2017;242:609–15. Copyright 2017, Elsevier.

FIG. 11.10 Schematic representation of the colorimetric method for the detection of NO
2. Reproduced with permission from Ye Y, Guo Y, Yue Y, Zhang Y. Facile colorimetric detection of nitrite based on anti-aggregation of gold nanoparticles. Anal Methods 2015;7:4090–6. Copyright 2015, the Royal Society of Chemistry.

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amino groups in the solution, and then decrease the aggregation of AuNPs [50]. Song et al. proposed an antiaggregation method for the detection of SCN
. The mechanism is as follows. The presence of CTAB in AuNP solution resulted in the aggregation of AuNPs with a red-to-blue color change. The addition of SCN
interrupted the aggregation of AuNPs since it showed better affinity toward AuNPs than that of CTAB. A turnover color change from blue to red was observed. [51] Chen’s group described a method that was based on the antiaggregation of AuNPs to quickly detect I
[52]. As depicted in Fig. 11.11, Hg2+ can be used as a cross-linking agent to induce the aggregation of thymidine functionalized AuNPs. However, in the presence of I
, Hg2+ preferentially reacted with I
to form HgI2 instead of reacting with thymidine. The color of the solution changed from blue to red dependent on the concentration of I
. Lu et al. proposed an antiaggregation colorimetric method for the detection of OCl
by making full use of the strong oxidizing properties of OCl
[53]. As displayed in Fig. 11.12, AuNPs can be gathered together by the DTT containing two thiol groups. In the presence of OCl
, the thiols were oxidized to sulfinic acid or disulfide bonds, which were unable to be chemiadsorbed onto the surface of AuNPs. Then, the AuNPs were transformed into unassembled forms, and the color of the solution became red.

FIG. 11.11 A scheme displaying the detection of iodide based on the antiaggregation of modified AuNPs. Reproduced with permission from Chen L, Lu W, Wang X, Chen L. A highly selective and sensitive colorimetric sensor for iodide detection based on anti-aggregation of gold nanoparticles. Sens Actuator B: Chem 2013;182:482–8. Copyright 2013, Elsevier.

ETCHING-BASED COLORIMETRIC METHODS

341

FIG. 11.12 Schematic representation of the hypochlorite sensor with unmodified gold nanoparticles. Reproduced with permission from Lu L, Zhang J, Yang X. Simple and selective colorimetric detection of hypochlorite based on antiaggregation of gold nanoparticles. Sens Actuator B: Chem 2013;184:189–95. Copyright 2013, Elsevier.

DETECTION OF SMALL ORGANIC MOLECULES Foroogh et al. presented a method for the analysis of catechol based on unmodified AuNPs and 4-mercapto-phenylboronic acid (4-MPBA) [54]. In the absence of catechol, the addition of 4-MPBA resulted in the aggregation of AuNPs due to the self-dehydration condensation of the boronic acid groups. In the presence of catechol, boric acid-diol binding is stronger than self-dehydration condensation, thereby alleviating the aggregation of AuNPs. The color changed from blue to red, which can be used for the detection of catechol. The proposed colorimetric detection method exhibits the advantage of eliminating the need for AuNP-surface modification, avoiding cumbersome separation processes and enabling rapid determinations.

ETCHING-BASED COLORIMETRIC METHODS As introduced above, aggregation or antiaggregation-based methods have gained much attention for on-site detection of environmental pollutants. However, these methods often require an NPs’ functionalization step, making them time-consuming and cumbersome to operate. In addition, they always suffer from being aggregated under high concentration of salt. In order to avoid the above shortcomings, colorimetric methods based on target-induced etching of NPs or the formation of new nanostructures have become a research hotspot and have been used to determine some of the target, including

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Cr(VI), Co2+, Cu2+, Fe3+, Pb2+, CN
, I
, Cl
, S2
, NO
2 , and HCHO. The LSPR of the PMNPs is determined by their size, morphology, interparticle distance, etc. Etching of PMNPs can lead to changes in their morphology and size, causing variation in LSPR uptake. For this method, unmodified PMNPs can be used directly for the detection of target analytes, thereby avoiding the tedious labeling process. So far, the analytes capable of being detected using this method can be mainly divided into three types: (1) under certain conditions, oxidative potentials of the analytes are higher than that of Au+/Au or Ag+/Ag (such as H2O2, Cu2+, Cr6+, I2, Fe3+, and NO
2 ); (2) it is capable of producing a metal-Au alloy or a stable Au/analyte complex so as to reduce the electrode potential of Au+/Au (e.g., Hg2+, CN
, and Cl
); and (3) it can trigger the generation of strong oxidizing agents such as H2O2, hydroxyl radical (OH), and superoxide radical (O2
) (e.g., Co2+).

DETECTION OF METAL IONS A colorimetric approach for detecting trace Cr(VI) in aqueous solutions was developed using Ag@Au core-shell NPs. This method relies on the fact that Cr(VI) is able to etch gold and silver in the presence of hexadecyl trimethyl ammonium bromide (CTAB). The standard electrode potential of Au+/Au and Ag+/Ag is 1.691 and 0.7966 eV, respectively, which are decreased to 0.959 and 0.07133 eV when the



Br
of CTAB acts as a ligand (AuBr
2 + e ! Au + 2Br , E ¼ 0.959 eV [55] and AgBr + e ! Ag + Br , E ¼ 0.07133 eV). The standard electrode potential of Cr(VI)/Cr(III) (1.33 eV) is higher than that of Au+/Au and Ag+/Ag, enabling Cr(VI) to oxidize Ag@Au core-shell NPs. Upon the addition of Cr(VI), the gold shell and silver core were sequentially dissolved, along with a color change from wine red to yellow and finally to colorless. [56] Motivated by this idea, a silanization-titanium dioxidemodified filter paper trapping AuNPs was used for the colorimetric detection of Cr (VI) [57]. Zhang et al. introduced a Co2+-mediated etching of AuNRs based on Fenton-like reaction and used it for the visual detection of Co2+. As shown in Fig. 11.13, with the assistance of bicarbonate 2+ can initiate a Fenton-like reaction generating O2
, which will further etch AuNRs (HCO
3 ), Co

FIG. 11.13 Schematic illustration of Fenton-like reaction-mediated etching of AuNRs induced by Co2+. Reproduced with permission from Zhang Z, Chen Z, Pan D, Chen L. Fenton-like reaction-mediated etching of gold nanorods for visual detection of Co2+. Langmuir 2015;31:643–50. Copyright 2015, American Chemical Society.

ETCHING-BASED COLORIMETRIC METHODS

343

in the presence of SCN
. The AuNRs will be selectively etched along the longitudinal direction, accompanied by an obvious color change from green to red [58]. In the presence of thiosulfate, Cu2+ can leach Au or Ag since the oxidative potential of Cu2+/Cu 3
3
(S2O3)5
3 is larger than that of Au(S2O3)2 and Ag(S2O3)2 . Based on this fact, Chen’s group developed methods based on Ag@Au core-shell NPs and Au@Ag core-shell NRs for the colorimetric detection of Cu2+ [59,60]. For the analysis of Fe3+, AgNPs were prepared using N-acetyl-L-cysteine (NALC) as protecting ligand. Once Fe3+ was added into the NALC-AgNP solution, the AgNPs were dissolved by Fe3+, giving rise to the gradual decrease of LSPR intensity [61]. Zhu et al. developed a colorimetric sensing method for detecting Pb2+ based on the Pb2+-accelerated etching of AuNRs. In this method, the sodium thiosulfate-induced dissolution of AuNRs was accelerated due to the Pb2+-induced formation of a monolayer of AuPb2 and AuPb3 alloys on the gold surface, leading to an obvious decrease in the electrode potential. The shape of the AuNRs was changed to nanospheres, accompanied by blueshift and vanishment of the LSPR peak of AuNRs [62].

DETECTION OF ANIONS Among all anions, cyanide is the most common one being studied since it is highly nucleophilic and can form very stable complex with metals. Taking advantage of this property, many colorimetric methods have been developed for cyanide sensing [63,64]. Tseng et al. developed a colorimetric approach using polysorbate 40 (PS-40)-stabilized AuNPs in the presence of high concentration of salt (Fig. 11.14) [64]. In the absence of cyanide, the PS-40 stabilized AuNPs was stable in the solution with high concentration of salt. By contrast, the addition of cyanide can remove the PS-40 away due to its stronger affinity toward gold. The loss of stabilizer caused the aggregation of AuNPs in high concentration of salt. Cheng et al. proposed a similar approach for the fluorescent and colorimetric detection of cyanide using PS-20 stabilized AuNPs [65]. An ensemble by incorporating the adenosine triphosphate (ATP)-stabilized AuNPs with Cu2+phenanthroline complexes was proposed for cyanide sensing [66]. The sensing mechanism of the ensemble was proposed as follows. Cyanide can complex with the Cu2+ in Cu2+-phenanthroline complex, and the released phenanthroline induced the bridging of AuNPs, leading to a color change of the AuNP solution from red to blue. Monometallic AuNPs and AgNPs were employed for the colorimetric

FIG. 11.14 Scheme of cyanide sensing using PS-40 stabilized AuNPs in the presence of salt. Reproduced with permission from Liu C-Y, Tseng W-L. Colorimetric assay for cyanide and cyanogenic glycoside using polysorbate 40-stabilized gold nanoparticles. Chem Commun 2011;47:2550–2. Copyright 2011, the Royal Chemical Society.

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sensing of cyanide, and the color is changed from red or yellow to colorless, which is not visually insensitive to naked eyes [67,68]. To overcome this disadvantage, Zeng et al. used Au@Ag and Ag@Au core-shell NPs as probes [69,70], which provide better visual resolution due to the use of core-shell structures. Recently, cyanide was found to be able to selectively etch the transverse faces of AuNRs, which is due to the less surface passivation and/or higher reaction activities at the end of AuNRs. Such etching process resulted in a decrease in the AuNR aspect ratio, and thus, a blueshifted SPR absorption and color change from peacock blue to pink [71]. Triangular silver nanoprisms (AgNPRs) were synthesized for the colorimetric sensing of ClO
. Upon the successive addition of the increasing concentrations of ClO
to AgNPRs, the absorption intensity of AgNPRs at 716 nm showed a gradual decrease with a significant blueshift. This phenomenon was accompanied by simultaneous enhancement in the absorption intensity near the in-plane quadrupole band at 470 nm along with a rapid color change of the AgNPR solution from blue to yellow [72]. In
the presence of CTAB, nitrite (NO
2 ) was able to react with gold to form [Au(Br)2] and thus changed the morphology of the AuNRs or Ag@Au core-shell NPs, causing LSPR peak shift and corresponding color changes [73,74]. Zhang et al. demonstrate a colorimetric avenue for iodide recognition and sensing using Cu@Au core/shell NPs. By converting the irregularly structured NPs to nearly spherical ones, the introduction of iodide induced an appreciable color change from purple to wine red [75]. As an extension of their work, the same group developed a colorimetric method for the analysis of S2
based on the inhibition of I
-induced Cu@Au core-shell NP morphology transformation. Without S2
, I
can induce an observable color change of the solution from purple to red as demonstrated above. By contrast, the transformation was significantly inhibited in the presence of S2
owing to its higher binding affinity toward gold than that of I
[76]. It is documented in textbooks that gold can be dissolved by aqua regia, which contains concentrated nitric acid and hydrochloric acid with a volume ratio of three. Benefiting from this property, Han’s group developed a colorimetric route for the detection of hydrochloric acid using unlabeled AuNPs in the presence of strong oxidant like HNO3 or H2O2. The hydrochloric acid-induced etching process caused a remarkable LSPR damping of the AuNP dispersion [77].

TARGET-INDUCED FORMATION OF NEW NANOSTRUCTURES DETECTION OF METAL IONS Anandhakumar et al. exploited the Hg2+-Au+ interaction chemistry to develop a colorimetric method for the detection of Hg2+ [78]. They stated that the role of Hg2+ is similar to that of gold seed, which can catalytically reduce Au+ to Au0 in the presence of ascorbic acid, changing the color from colorless to wine red. Wu et al. proposed a colorimetric method for the detection of Hg2+ using thioacetamide functionalized AuNPs [79]. It is based on the finding that a layer of HgS quantum dots was in situ coated onto the AuNPs to form Au@HgS core-shell nanostructures (Fig. 11.15). The LSPR was changed from Au core to HgS shell, which produced a color change from wine red to purple. Wang et al. put forward a method for the determination of Hg2+ using mesoporous silica-coated AuNRs coupled with ascorbic acid [80]. The introduction of Hg2+ into the detection system was reduced by the ascorbic acid to form Hg0 onto AuNRs through the mesopores. The amalgam was formed and the dielectric constant change of the surrounding mediums induced a LSPR shift and color change

TARGET-INDUCED FORMATION OF NEW NANOSTRUCTURES

SH2

H2S H2S AuNPs

C2H5NS H+

SH2

HgS

SH2 AuNPs

H2S H2S

HgS Hg2+

SH2

SH2

HgS AuNPs

HgS

SH2 H2S

345

HgS HgS

SH2

FIG. 11.15 Schematic illustration of Hg2+ sensing using thioacetamide functionalized AuNPs. Reproduced with permission from Zhang F, Zeng L, Yang C, Xin J, Wang H, Wu A. A one-step colorimetric method of analysis detection of Hg2+ based on an in situ formation of Au@HgS core-shell structures. Analyst 2011;136:2825–30. Copyright 2011, Elsevier.

of AuNRs. Inspired by this strategy, Li et al. developed a colorimetric method for the detection of Hg2+ using unmodified Au@Ag core-shell NPs, in which Hg2+ was reduced into Hg0 by Ag shell and coated on the Au core to generate Au-Hg alloys inducing the aggregation of NPs. The resulting color changes of the solution from yellow to purplish red were dependent on the concentration of Hg2+ [81]. A colorimetric approach involving the use of AuNPs and hydroquinone was established for the detection of Ag+. The sensing mechanism is based on the finding that Ag+ was reduced by hydroquinone to deposit a silver shell onto the surface of AuNPs, resulting in a color change from red to yellow. However, the Hg2+ interfered the detection since it can induce the aggregation of AuNPs and lead to a color change from red to blue. Similarly, Ag+ can be readily detected using ascorbic acid as a reducing agent and mesoporous silica-coated AuNRs as probes [82]. Sadollahkhani et al. developed a colorimetric assay for Cu2+ using ZnO@ZnS core/shell NPs [83]. As shown in Fig. 11.16, the principle of this method is that Cu2+ can react with ZnS to generate a new ZnO@CuS core/shell NPs (ZnS + Cu2+ ¼ CuS + Zn2+). The variation of the composition of the shell caused the color of the solution to change from colorless to yellow. The detection system was immobilized onto papers as disposable sensors. Cu2+

Zn2+

ZnO

ZnO

ZnS

CuS

ZnO@ZnS NPs

ZnO@CuS NPs

Sensing mechanism

FIG. 11.16 The colorimetric assay for Cu2+ using ZnO@ZnS core/shell NPs. Reproduced with permission from Sadollahkhani A, Hatamie A, Nur O, Willander M, Zargar B, Kazeminezhad I. Colorimetric disposable paper coated with ZnO@ZnS core-shell nanoparticles for detection of copper ions in aqueous solutions. ACS Appl Mater Interfaces 2014;6:17694–701. Copyright 2014, American Chemical Society.

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DETECTION OF ANIONS The research group of He proposed a high-throughput strategy for the detection of S2
(H2S) using Au@Ag core-shell NRs and NPs as molecular probes [84,85]. As displayed in Fig. 11.17, this method is based on the analysis of color variation of the individual NRs and NPs generated due to the transformation of the shell composition from Ag to Ag2S. To collect the scattering light emitted from each NP, a single-particle spectral dark-field microscope with a wavelength resolution of 1.73 nm per pixel was employed. This method is highly sensitive as it can obtain the signal from each NP. Zeng et al. developed a colorimetric assay for sensing I
by coupling Au@Ag core/shell NPs with 2+ Cu . When I
was exposed to the mixture of Au@Ag core/shell NPs-Cu2+, Cu2+ can oxidize I
into I2, (2Cu2+ + 4I
¼ 2CuI + I2), which can sequentially oxidize silver shells to form AgI (I2 + 2Ag ¼ 2AgI). With the addition of the incremental concentration of I
, the color changed from red to purple, which can be used for the semiquantitative detection of I
. The Au@Ag core/shell NPs-Cu2+ detection system was also immobilized into agarose gels, which have realized their application in the semiquantitative determination of I
in dried kelps. [86] Taking advantage of the same strategy, a colorimetric method for the detection of I
using Au@Ag core-shell nanobipyramids was presented. The mechanism also relies on the reaction that Cu2+ could oxidize I
into I2, which sequentially etches the Ag shell of the Au@Ag core-shell nanobipyramids, resulting in the blueshift and intensity decrease of the longitudinal LSPR peak, which can be utilized to quantitatively detect the concentration of I
in the range of 1.0–15 M [87].

DETECTION OF SMALL ORGANIC MOLECULES Silver mirror reaction is typically utilized for the manufacture of mirror and thermos bottle. Zeng’s group developed a colorimetric method for the detection of formaldehyde using AuNPs assisted by silver mirror reaction [88]. As shown in Fig. 11.18, when the mixture of AuNPs and Tollen’s reagent was exposed to formaldehyde, silver mirror reaction occurred, and the resulting silver was wrapped onto the surface of AuNPs, leading to the formation of Au@Ag core-shell structures. The LSPR of the solution was changed from 520 to 400 nm accompanied by the color that changes from red to yellow, depending on the concentration of formaldehyde. This method is highly sensitive, and the detection limit is as low as 50 nM. Such a good sensitivity is owing to the fact that the LSPR of Au@Ag core-shell NPs is closely related to the thickness ratio of the silver shell to the gold core, which can be affected by a small amount of formaldehyde. The researchers further encapsulated the test system in [H2S] Au@Ag

Au@Ag2S

FIG. 11.17 Schematic illustration of colorimetric detection of H2S using Au@Ag core-shell NPs. Reproduced with permission from Hao J, Xiong B, Cheng X, He Y, Yeung ES. High-throughput sulfide sensing with colorimetric analysis of single Au–Ag core–shell nanoparticles. Anal Chem 2014;86:4663. Copyright 2014, American Chemical Society.

REFERENCES

347

FIG. 11.18 The schematic diagram of HCHO sensing using AuNPs assisted by silver mirror reaction. Reproduced with permission from Zeng J-b, Fan S-g, Zhao C-y, Wang Q-r, Zhou T-y, Chen X, et al. A colorimetric agarose gel for formaldehyde measurement based on nanotechnology involving Tollens reaction. Chem Commun 2014;50:8121–3. Copyright 2014, Royal Society of Chemistry.

agarose gels and constructed a portable “test strip” for formaldehyde detection. The method is simple, fast, sensitive, and selective and is possible to realize the indoor formaldehyde monitoring. Li et al. adopted the same strategy for the detection of aldehydes, and they monitored the scattering spectra of a single AuNP to lower the detection limit [89].

SUMMARY AND OUTLOOK In summary, PMNPs have been widely utilized as optical probes for the colorimetric detection of environmental pollutants including heavy metal ions, toxic anions, and some organic molecules. These methods are sensitive and simple to handle. More importantly, they allow for on-site monitoring of the concentration of pollutants, which is highly beneficial for the rapid assessment of environmental pollution. In the future, we predict that firstly, this particular area will focus on the development of PMNPs with other morphologies like disks, wires, hollow structures, and branched particles that will enable the selective and sensitive detection of target analytes with enhanced stability and better visual resolution. Secondly, the development of solid-based medium to effectively immobilized NPs as test papers or chips will be another possible trend. Thirdly, immobilization of different detection systems into one chip is expected to be the advantages for the simultaneous detection of multiple pollutants one at a time.

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