Gold nanoparticle-based optical sensors for selected anionic contaminants

Accepted Manuscript Gold nanoparticle-based optical sensors for selected anionic contaminants Cheng Fang, Rajarathnam Dharmarajan, Megharaj Mallavarapu, Ravi Naidu PII:

S0165-9936(16)30238-2

DOI:

10.1016/j.trac.2016.10.008

Reference:

TRAC 14842

To appear in:

Trends in Analytical Chemistry

Received Date: 1 September 2016 Revised Date:

13 October 2016

Accepted Date: 20 October 2016

Please cite this article as: C. Fang, R. Dharmarajan, M. Mallavarapu, R. Naidu, Gold nanoparticle-based optical sensors for selected anionic contaminants, Trends in Analytical Chemistry (2016), doi: 10.1016/ j.trac.2016.10.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

Gold nanoparticle-based optical sensors for selected anionic contaminants

2

Cheng Fang1,2*, Rajarathnam Dharmarajan1,2, Megharaj Mallavarapu1,2 and Ravi Naidu1,2

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1

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(CRC CARE), University of Newcastle, Callaghan NSW 2308, Australia

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2

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2308, Australia

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Abstract: Nano-sensors have received intensive attention due to their unique sensitivity and selectivity,

8

which mainly originate from modifications and reactions that occur at nano scales. An important

9

ingredient for these sensors are gold nanoparticles which have been developed as the core material to

10

detect anions, in particular in contaminated water. Especially these nano-sensors find specific

11

applications for selected harmful anions which include a controlled anion F-, toxic contaminants CN-

12

and AsO33-/AsO43-, and anionic fluorosurfactants. The anionic fluorosurfactants mainly include

13

perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), both of which have recently

14

been listed as emerging contaminants and categorised as persistent organic pollutants. This review

15

addresses recent advancements in the development of gold nanoparticle based nano-sensors for such

16

anions and their performance limitations towards on-site applications.

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Global Centre for Environmental Remediation (GCER), University of Newcastle, Callaghan NSW

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Cooperative Research Centre for Contamination Assessment and Remediation of the Environment

Keyword: Gold nanoparticle, sensor, fluoride, cyanide, arsenic, fluorosurfactants, on-site applications

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Abbreviations: AuNP, gold nanoparticle; PFAS, Poly- and perfluoroalkyl substance; PFOS,

21

perfluorooctane sulfonate; PFOA, perfluorooctanoic acid PFOA; AFFF, aqueous film-forming foam;

22

LOD, limit of detection; SAM, self-assembled monolayer; QD, quantum dot; DLS, dynamic light

23

scattering; HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; DMF, dimethylformamide;

24

PBS, phosphate-buffered saline buffer;

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polyvinypyrrolidone; GSH, glutathione; DTT, DL-dithiothreitol; Cys, cysteine; PDCA, 2,6-

26

pyridinedicarboxylic acid; MOPS, 3-(N-morpholino)propanesulfonic acid; RS, resonance scattering;

27

CTAB, hexadecyltrimethylammonium bormide; ppt, part per trillion; ppb, part per billion; ppm, part

28

per million; FRET, fluorescence resonance energy transfer; AuNC, gold nanocluster; BR buffer,

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Britton-Robinson buffer; AgNP, silver nanoparticle; MIP, molecular imprinting polymer; ECL,

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electrochemiluminescence;

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indicated or applied. USEPA, US environmental protection agency; WHO, world health organization.

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ethylenediaminetetraacetic acid; PVP,

PPARα, peroxisomal proliferator-activated receptor-alpha. NIL, not

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EDTA,

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1. Introduction

40

1.1. Anion contamination

41

Water is the origin of life and a key substance for the health of our planet. Nevertheless, it is estimated

42

that over 10 million tonnes of toxic chemicals are released into aquatic environments every year as a

43

consequence of anthropogenic activities [1-3]. New chemicals are constantly being produced, and it is

44

essential that the dynamics of these chemicals / contaminants in the environment are assessed in order

45

to quantify their risks to human and environmental health.

46

Furthermore, ongoing research continues to improve our understanding of past and emerging

47

contaminants. Consequently, concerns about water contamination have attracted increasing attention

48

[4], which can be evidenced from the recent virtual issue of “water analysis for emerging chemical

49

contaminants”[5], and biennial reviews “water analysis: emerging contaminants and current issues” [6,

50

7].

51

For water contamination, monitoring anions in particular, along with surfactants, represent a large

52

family of contaminants and pose a major challenge. They are generally crucial to physiological

53

function as well as to various industrial processes. Anions can be either essential to sustained growth,

54

act as harmful contaminants (such as CN- and AsO33-/AsO43-), or regulate to control the concentration

55

level (such as F-). Herein, we will review recent sensor developments using nanoparticles for the

56

detection of anions, including a controlled anion (F-), a harmful contaminant (CN-), an extremely

57

toxic contaminant (AsO33- or AsO43-) and a family of anionic fluorosurfactants. Among them, F- is

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encountered by us daily, CN- and AsO33-/AsO43- are of high risk whilst fluorosurfactants are bio-

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accumulative, highly persistent and detected globally.

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ACCEPTED MANUSCRIPT 1.2. Gold nanoparticles (AuNP) - based sensors

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The standard testing methods for anions involve chromatography [8], electrophoresis [9], ion-

62

selective electrode [10], atomic absorption spectrum, ICP-MS etc. [6, 7]. However, these are generally

63

expensive and time consuming and further are normally unsuitable for on-site field applications. For

64

on-site applications there have been several attempts to develop sensors which help to pre-screen the

65

potential anionic contaminants rapidly [8, 10, 11].

66

For anion sensors, different types of organic-dye types were developed based on visible color changes.

67

These can be classified into several categories such as NH-based hydrogen bonding dyes, Lewis acid-

68

base dyes, metal-ion-templated dyes and transition metal complex dyes [12-14]. The basic concept is

69

to convert the binding event or recognition phenomena to optical signals for detection [15]. Usually

70

these organic dyes are synthesised and dissolved in organic solvents. Along with their stability

71

concerns, there are other drawbacks, such as toxic concerns, specificity and sensitivity concerns,

72

which might restrict their application, particularly for on-site environmental monitoring.

73

Consequently, inorganic dyes, such as quantum dots (QTs), nanoparticles (NPs) and nanoclusters [16,

74

17] [18] have achieved significant success recently.

75

Amongst nanoparticle developments gold nanoparticles (AuNPs) are the most stable nanoparticles so

76

far with features of having a large surface area, strong adsorption and stability, which enable them to

77

be functionalised for selective testing. Due to their nano size, the functionalisation can be carried out

78

at nearly molecular-level, which makes them suitable for ultrasensitive detection. Other highlights

79

include a unique shape, size-dependent optical properties, hyper-quenching ability for fluorescence,

80

and surface-enhanced Raman scattering (SERS) capability [19-22]. More importantly

81

absorption is located in the visible region whilst devoid of any proven toxicity [23, 24].

82

Optical properties, when dominated by the surface plasmon or inter-particle plasmon, are drastically

83

influenced by the surface environment of the nanomaterials, which offer a path for making tools to

84

monitor the chemicals and other substances if they enable the modification of the surface environment

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AuNP’s

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ACCEPTED MANUSCRIPT [20-22]. For example, based on the AuNP aggregation that is tuned by the molecular recognition

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event, a detection limit of 50 fM (10-15 M) using UV-vis spectrometer and 10 pM (10-12 M) by the

87

naked eye has been reported [25].

88

Several types of optical sensors were fabricated using AuNP as the test platform for anions [23, 24].

89

The recognition event for targeted anions include induced aggregating, anion binding, releasing (of

90

dye or others) and etching of AuNP as well (Figure 1). In certain instances bio-recognition might also

91

be involved, such as the pregnancy test, which has been in practice for decades. When the native

92

inherent optical properties of AuNP cannot feature the detection capability to produce an observable

93

signal, a dye or probe can be introduced into the matrix to make it possible, as listed in Table 1. The

94

recognition events by these materials and hence the type of sensing materials are classified based on

95

phenomena such as (i) colorimetric, which utilise the color changes of the AuNP matrix solution and

96

which is usually accompanied with UV-vis [25]; (ii) scattering, including elastic scattering (without

97

wavelength change from the stimulation illumination), inelastic scattering (with wavelength change,

98

such as Surface Enhanced Raman Scattering (SERS)) and dynamic light scattering (DLS) and (iii)

99

fluorescence through the emission after being excited. However, for SERS and fluorescence testing

100

specific setups and devices are needed, which might inhibit their on-site applications. Though there

101

are various other optical detection methods, such as infrared spectroscopy, luminescence spectroscopy,

102

surface plasmon resonance, etc., detailing those is out of the scope of the present review which

103

confines to the optical sensing methods with AuNP’s direct contribution.

104

Furthermore, for the application through fluorescence detection, AuNP can easily quench the

105

fluorescence due to fluorescence resonance energy transfer (FRET) [23, 24]. FRET is a non-radiative

106

process where an excited state donor (such as dye) transfers energy to a proximal ground state through

107

long-range dipole-dipole interactions with AuNP as acceptor. Therefore, the fluorescence will be

108

quenched on the AuNP surface unless the fluorophore can be replaced and released into the solution,

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which is the general approach for the AuNP-based FRET test, as shown in Figure 2.

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Figure 1. Schematic drawing of the AuNP-based sensing process.

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Figure 2. Schematic drawing of FRET. Fluorophores are released due to replacement or etching.

Table 1. Matrix of AuNP-based sensors depending on detection approaches and dye.

Detection

Example

Without dye

With dye

approach

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ACCEPTED MANUSCRIPT Colorimetry

Color change due to

Dye enhances the color

reading

surface plasmon, such change as aggregation

or UV/vis Target’s Raman

Dye as Raman probe,

signal

aggregation induced

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Scattering

enhancement No signal except gold

FRET

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Dye as fluorophore,

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Fluorescence

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1.3. Sensing Anions: F-, CN-, AsO33-/AsO43- and PFOS / PFOA

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Figure 1 depicts the AuNP-based sensing process which differs from the dye-based approach. In the

120

latter case, for example, the selective detection of F- and CN- are usually based on their high negative

121

charge density, which brings about the strong H-bond with the –NH– (and similar) groups in the

122

sensing dyes [12-14]. In the former case, F- may be detected through the dye based sensors and CN-

123

can be detected through the coordination capacity of CN- which enables it to etch and dissolve notable

124

metals including AuNP. ‘Anions’ is rather a big family of compounds and the present review is

125

pertained to toxic and harmful anionic species enabling them to be screened in a rapid manner through

126

novel AuNP based sensing techniques.

127

Apart from these inorganic anions a family of organic anionic substances, poly- and perfluoroalkyl

128

substances (PFASs), have received much attention in the past decade due to their persistent nature and

129

suspected toxicities to human health and the ecosystem. PFASs have found many applications and are

130

the key ingredients of aqueous film-forming foams (AFFFs) acting as oxygen suppressing surfactants

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and are commonly used in fire-fighting [2, 3]. Unfortunately their widespread use in fire-fighting, as

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ACCEPTED MANUSCRIPT well as the emissions from other sources like fluoropolymer manufacture, ammonium

133

perfluorooctanoate and fluoropolymer dispersion, has led to their global distribution and accumulation

134

in the environment due to their inert fluorocarbon skeleton [26-29]. Specific attention has been given

135

to perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) which have been formulated

136

in AFFFs as fluorosurfactants and are known to have entered ecosystems and food chains as anions.

137

Thus their continuous monitoring has become routine with many regulatory authorities and

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developing improved sensor materials for them has gained increased attention as well. 2. AuNP-based sensors for fluoride (F-) ions

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Fluoride (F-) is the smallest anion with high charge density and hard Lewis base nature, making it

141

unique compared to other anions. Water fluoridation has beneficial effects to dental caries, contrarily

142

its excessive intake give rise to dental / skeletal fluorosis, osteoporosis, and inhibition of

143

neurotransmitter biosynthesis in foetuses [30]. The maximum daily intake concentration is

144

recommended at 4 ppm (part per million) by the US environmental protection agency (USEPA).

145

USEPA has approved use of the ion-selective electrode [31] and SPADNS Zirconium [32] for F-

146

detection. The latter method is based on the reaction of F- with a dark red zirconium-dye complex,

147

where F- combines with the zirconium ion and detaches it from the SPADNS-zirconium to form a

148

colorless zirconium-fluoride complex. The subsequent colorimetric change is monitored. A similar

149

approach, using zirconium xylenol orange reagent as dye, has been combined with a smartphone for

150

an on-site test in the range of 0-2 ppm [33].

151

When the AuNP surface is functionalised with organic molecules, the presence of F- might induce the

152

AuNP to aggregation leading to a color change, which can be employed for F- detection, as shown in

153

Table 2. For example, thio-glucose capped AuNP is among the first generation of AuNP-based

154

colorimetric methods for detection of F- [34]. Upon the addition of F- into the solution, a red-shift in

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the surface plasmon was observed. The molar extinction ratio at 523 nm (corresponding to free AuNP)

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and 800 nm (corresponding to aggregated AuNP) was used to analyse the concentration of F-. Another

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example is that of an isothiouronium-based AuNP probe [35] which showed a selective coloration for

158

hydrophobic anions in water due to the anion exchange and the AuNP aggregate. In a series the

159

response was found in the order PF6- > SCN- > I-. For F- detection, however, a mediator of 3-

160

nitrophenylboronic acid was needed to form a trifluoroboronate anion (-BF3-) prior to the test.

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aggregate process, such as the Si-O-Si bond breakage by F-. A probe was thus fabricated by anchoring

164

4-mercaptopyrdine on AuNP, and further assembling 3-aminopropyltrimethoxysilane to form a thin

165

Si-O-Si protecting layer to encapsulate the AuNP [36]. In the presence of F-, the Si-O-Si bond will be

166

broken so the protecting layer is destroyed, as shown in Figure 3. Consequently, the aggregation of

167

AuNPs gives rise to significant color change in aqueous solution. Similarly, F- can detach silyl

168

moieties from phenol groups on the surface of an AuNP probe [37] causing a rearrangement reaction,

169

which is accompanied by the release of a dithiol in a spontaneous and irreversible reaction in aqueous

170

solution. The release of dithiol causes the aggregation of AuNP leading to noticeable color change.

171

However, it should be pointed out that the Si-O-Si bond can also be broken by the presence of OH-,

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particularly in a strong base solution. Consequently, the pH value of solution must be controlled for

173

the test, as shown in Table 2.

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Due to its unique nature the strong H-bond arising from interaction with F- has been used to develop

176

sensor materials. For example, a thiobarbituric acid-capped AuNP sensor has been reported [38]

177

where the color change associated with F--induced aggregation has been successfully used for the bare

178

eye detection of F- at 10 mM. Another case is a thiourea based receptor, which is linked to

179

acridinedione dye for a modified AuNP for F- detection [39]. Upon selective recognition of F- in an

180

acetonitrile medium, the fluorescence was quenched due to the H-bonding and the deprotonation of

181

the thiourea group.

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182 183

Because AuNP is a good quencher for the fluorescence process, FRET has been employed to build the

184

fluorescent sensor for F- detection [40]. In this case, a fluorophore is needed in the assembly, such as a Page 9 of 44

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quantum dot (QD). An example of this is thioglycolic acid modified QDs, which were linked to a

186

citrate capped AuNP surface through H-bond. F- can disassemble the link of QD-AuNP resulting in

187

the release of QDs with subsequent fluorescent recovery. Similarly, the strong affinity between

188

boronate (Lewis acid) and F- (Lewis base) also leads to the development of a FRET sensor [41],

189

wherein

190

phenylborinic acid and diol. In the presence of F-, the boronate ester was converted to trifluoro borate,

191

which broke the linkage and released the QDs from AuNP with consequent fluorescent recovery of

192

QD.

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QD was fixed on the AuNP surface through the formation of cyclic esters between

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The question is, if the dye itself shows a colorimetric reaction with F- (not the coloration of AuNP,

195

such as aggregates), what is the contribution from AuNP in the sensing mechanism? For example, in

196

the absence of AuNP, 4-mercaptophenylboronic acid acted as the quencher whilst carbon nanodots

197

acted as a fluorophore in a detection process for F- [42]. A similar report on 2,1,3-benzothiadiazole

198

also showed its ability in detecting F- in the absence of AuNP [43]. That is, the AuNP’s contribution

199

towards sensing processes such as color enhancement, dye-loading matrix, stability in aqueous

200

solution, nano platform for detection etc., can’t be totally ignored or discarded, given that AuNP’s

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participation helps to improve the detecting ability of the materials. This is clearly reflected in the

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detection of cyanide ion, which is an entirely different type of anion than F-.

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ACCEPTED MANUSCRIPT Figure 3. Schematic drawing of the response based on the Si-O bond cleavage induced by F-.

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Reproduced with the permission from [36]

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Table 2. AuNP-based F- sensor. Signal/Detector

Solution

Mechanism

Working range

Absorption at 523 nm 10

AuNP

/800 nm, or naked eye

mM AuNP aggregate

10-50

mM NIL

SC

SAM-

LOD

RI PT

Matrix

Comments

Ref.

Ambiguity in the [34]

(190-950

mechanism

ppm)

aggregation

M AN U

HEPES pH7.0

of

SAM-

Absorption at 525 nm 300 mM NaCl, Mediated anion exchange, 40-240 mM NIL

Mediator is needed [35]

AuNP

/640 nm, or naked eye

(740-4560

and Ambiguity in

ppm)

the mechanism of

AuNP aggregate

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pH5.5

aggregation

SAM-

Absorption at 525 nm 20 mM EDTA, O-Si-O breaking, AuNP 1-7 ppm

AuNP

/625 nm, or naked eye

SAM-

Absorption at 529 nm DMF/PBS,

AuNP

/800 nm, or naked eye

SAMAuNP

Naked eye

pH7.7

pH7-8

1 ppm

Spiked tap water [36]

aggregate

tested

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pH5-8

O-Si-O breaking, AuNP 120 µM-1.5 120

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aggregate

mM

µM No real sample.

[37]

(~2-30 (~2 ppm)

ppm)

H-bond forming, AuNP NIL

10

mM Qualitative test

aggregate

(190 ppm)

[38]

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PET,

fluorescence acetonitrile

H-bond

quench in the range of

forming, 50-700

fluorescence quenching

recovery at 520 nm QD-AuNP

FRET,

mM H-Bond

HEPES pH7.0

Fluorescence 10 mM PBS, B-O

recovery at 520 nm

pH7.4

breakage, 5-35

fluorescence recovering bond

µM NIL

(95-665 ppb)

breakage, 5-45

fluorescence recovering

Spiked tap water [40] tested

µM 50 nM (950 Spiked tap water [41]

(95-855 ppb)

ppt)

tested

and

biological imaging

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[39]

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Fluorescence 10

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FRET,

Organic media.

(~1-13 ppm)

410-460 nm QD-AuNP

µM NIL

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Dye-AuNP

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3. AuNP-based sensors for cyanide (CN-) ions

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The toxicity of cyanide ion (CN-) towards mammals is well known and to almost all other forms of

211

life too, due to its propensity to bind with iron in peroxidase, ferric haemoglobin and myoglobin,

212

catalase and cytochrome c oxidases [12, 44, 45]. The recommended upper limit concentration in

213

drinking water is 50 ppb in Europe, 4 ppb in Australia, 200 ppb from USEPA and 70 ppb from World

214

Health Organization (WHO), respectively[46].

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Commercially available AuNPs usually are covered with organic acids such as citric acid. When the

216

solution circumstance is changed, AuNPs can easily become aggregated. Recently, adenosine

217

triphosphate (ATP)-stabilised AuNP has been developed to increase the stability over a wide pH

218

range and even in a high salt concentration, which is attractive for applications in a physiological pH

219

range, such as for biological and environmental tests [44].

220

Most of the CN- sensors function through strong coordination interactions between CN- and metal

221

ions including Cu2+, Ag+ and Au3+. For example, a complex of ATP-AuNP-Cu-phenanthroline has

222

been developed [44] where, in the presence of CN-, Cu2+ was coordinated with CN- with subsequent

223

release of free phenanthroline resulting in AuNP aggregation with ensuing color change of the

224

solution (Table 3). However, it is to be noted that the coordination interaction is not specific for CN-

225

due to possible interferences from other ions with high coordination affinity towards Cu2+.

226

Another approach is the etching and dissolution of AuNP by the aggressive CN- [47]. In the presence

227

of oxygen, CN- can etch the inert gold via the formation of a complex of Au(CN)2+. There are not too

228

many other anions which can proceed through this etching process and thus a specific detection is

229

expected by change in color of AuNP colloids. In the fluorescence approach FRET is exploited

230

where a fluorophore is needed, such as Rhodamine B [48], imidazole-functionalised polyfluorene [47]

231

and fluorescein isothiocyanate (FITC) [49] [50] . Even a core-shell nanoparticle of silver/gold (silver

232

at the core) was developed [51]. Another approach is using gold nanoclusters (AuNC) rather than

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ACCEPTED MANUSCRIPT fluorophore [52]. In contrast with a nanoparticle that is a fluorescence quencher, metal nanoclusters

234

usually yield strong fluorescence and can be used to replace fluorophore [53].

235

In an effort to increase the sensitivity of CN- detection a two-photon excitation system has been

236

considered. The advantages include that excitation is carried out at the near-infrared energy region

237

with nil photo-bleaching and auto-fluorescence, resulting in enhanced signal-noise ratios and

238

improved sensitivity. These types of sensors have been developed not just to detect CN-, but also to

239

image CN- in plant issue [54]. The sensor assembles graphene quantum dots/AuNP conjugate, where

240

the former exhibits excellent two-photon properties whilst the latter acts as a good fluorescence

241

quencher. In a similar fashion an up-conversion nanoparticle has been developed to detect CN- [55],

242

however the nanoparticle is a rare-earth metal rather than AuNP.

243

An Au-based biosensor has also been reported for the detection of CN- where a horse-radish

244

peroxidase enzyme-modified electrode was used for inhibitive and selective determination of CN-

245

[56]. Since the process involves collection of an electrochemical signal the presence of AuNP

246

enhanced the electron transfer reaction between the electrode and the active site of the enzyme.

247

However, it is not a colorimetric detection.

248

A SERS sensor has also been developed for CN- [57] where the CN- bond’s vibration was directly

249

monitored through a Raman signal, as listed in Figure 4. The Raman signal of C-N stretch at 2154 cm-

250

1

251

ascorbic acid (which was coated on the AuNP) with CN-. A detection limit of 100 ppt (part per

252

trillion) was reached by this method. However a gradual etching of Au was also identified, although

253

this was slow and took hours at the low concentration of CN-.

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was significantly enhanced by the AuNP aggregate, which was achieved by the replacement of

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Figure 4. Schematic drawing of the SERS detection of CN- based on etching of AuNP. Reproduced

256

with the permission from [57]

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Table 3. AuNP-based CN- sensor. Signal/Detector

Solution

Mechanism

Working range

Absorption at 680 Aqueous at neutral Cu2+ nm

pH,

decomplexed, 10-25

AuNP aggregated

Comments

Ref.

µM 10 µM (260 Ambiguity

over [44]

SC

Cu-dye-AuNP

LOD

RI PT

Matrix

(260-650

ppb)

dye contribution

M AN U

ppb)

No incubation. dye-AuNP

Fluorescent at 402 Aqueous,

no FRET: AuNP etched, 0.3-130 µM 0.3 µM (~8 Spiked

nm (excitation at incubation.

fluorescence recovered

dye-AuNP

Fluorescent at 577 Aqueous,

Colorimetric

at 40

fluorescence recovered

EP

information available

FRET -AuNP

ppb)

sample tested

FRET: AuNP etched, 150 nM-45 80 nM (~2 Desorption

nm (excitation at incubation 520 nm)

(~8-3400

water [47]

ppb)

TE D

355 nm)

not

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mM

high concentration pH7.4,

30

level, fluorescence incubation.

µM

(~4- ppb)

1200 ppb)

Rhodamine identified

PBS, AuNP aggregated at 0.1-300 µM 0.1 µM (2.6 Dual min high level,

concentration (2.6-7800

of [48]

ppb)

functional [49]

system.

fluorescence ppb)

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at low level Fluorescent at 520 5

mM

sodium FRET: AuNP etched, 1-10

nm (excitation at phosphate,

60 min incubation.

(26-260 ppb)

ppb)

Ag@AuNP

Absorption at 520 Aqueous pH 10, 5 AuNP nm, naked eye

min incubation

decolored

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480 nm)

pH10, fluorescence recovered

µM 1 µM (26 Masking agent of [50]

RI PT

FRET -AuNP

recovered at low level

TE D

Fluorescent at 516 BR buffer pH10, 30 Two-photon-excitation nm (excitation at min incubation.

Bio-AuNC

Fluorescent at 640 NaOH-NaHCO3

(~4 ppb)

20

520 nm)

incubation.

Electrochemical

50

current

pH5.0,

mM

improved [51]

version compared to AuNP

1-200

µM 0.52

(26-5200

µM Clean background

[54]

(~14 ppb)

ppb)

Au nanocluster etched, 200 nM-9.5 200 nM (~5 pH12 might be [52]

min fluorescence

AC C

nm (excitation at pH12,

bio-AuNP

EP

780 nm)

interference from

µM An

ppb)

QD-AuNP

needed to avoid

I- and S2-

etched, 0.4-32 µM 0.16 (~10-830

S2O82-/Pb2+

diminished

NaAc Inhibition no response

µM

(~5- ppb)

limiting

250 ppb) aerometric 0.1-58.6 µM

the

application 0.03

µM Electrochemical

(2.6- (~0.8 ppb)

[56]

approach

Page 18 of 44

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incubation. Fluorescent at 650 NaOH-NaHCO3 nm (excitation at pH11, 370 nm)

Au nanocluster etched, 5-120

incubation fluorescence

information

(130-3120

not diminished

ppb)

SC

available

Raman signal of Aqueous pH11, no CN- leads to AuNP 110-4500 CN at 2154 cm-1

M AN U

AuNP

incubation.

aggregation

and ppt

µM pH11 might be [58]

(~5 ppb)

110 ppt

limiting

the

application

10 µL samples is [57] needed at pH11. AuNP

etching

takes hours at ppt level of CN-

AC C

EP

TE D

Raman enhancement

258

µM 0.19

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bio-AuNC

1500 ppb)

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4. AuNP-based sensors for arsenic ions (AsO33-/AsO43-)

260

Inorganic arsenic (As) might be among the most toxic contaminants and has been categorised as a

261

class A human carcinogen [59]. As the 20th most abundant element in the terrestrial crust, it exhibits

262

influence on 140 million people globally, including Bangladesh, India, China and the United States

263

etc.[59-61]. Long term exposure to arsenic contamination might lead to various types of cancers of

264

skin, lungs, bladder, kidney, etc. and other kinds of diseases such as diabetes, developmental and

265

reproductive problems, and cardiovascular disease. Consequently, USEPA and WHO have

266

recommended the maximum contamination level in drinking water at 10 ppb [59, 60].

267

Usually there are mainly two types of contaminants, pentavalent arsenic (arsenate or As(V)) and

268

trivalent arsenic (arsenite or As(III)), which usually exist as anions of AsO33-/AsO43-. Among them,

269

As(III) is generally targeted because it has been found to be more toxic to humans and have a higher

270

mobility in environment than As(V). The measurement approaches of arsenic as element include

271

atomic absorption spectroscopy, atomic fluorescence spectroscopy, atomic emission spectroscopy,

272

and electrochemical analysis [59, 60]

273

Unlike F- but similar with CN- [57], the characteristic bonded peaks of As(III) and As(V) can be

274

clearly identified and distinguished in Raman spectrum offering a direct SERS analysis [60]. For

275

example bands at 780-812 cm-1 and 410 - 430 cm-1 have been assigned to AsO43- whilst bands at 726

276

cm-1 and 439 cm-1 are assigned for AsO33- (Figure 5a) [62, 63]. However, with respect of SERS test,

277

usually silver NP or nanocrystal is preferred to AuNP. Furthermore, the Raman intensities of the

278

above peaks are weak necessitating the interferences been removed by sample-preparation with

279

enhancement of the signal through the incorporation of dye [61].

280

AuNP features an extremely high specific surface area, a property of which can be exploited to extract

281

arsenic from the aqueous phase even at trace levels to improve the sensitivity. In such a case the

282

surface property can be modified or functionalised to preferentially select arsenic anion. If this

283

process leads to the aggregate of AuNP, which is needed for SERS, the colorimetry (visible spectrum

AC C

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259

Page 20 of 44

ACCEPTED MANUSCRIPT or visible test) generally provides a good and economic detection platform, although the sensitivity is

285

compromised and not that comparable to SERS [60].

286

However, SERS may not be the only choice to address the sensitivity issue. An alternative detection

287

approach, dynamic light scattering (DLS) has been recently developed showing a high sensitivity

288

(Figure 5 (b)) for arsenic anion down to a LOD of 10 ppt, a value much lower than the recommend

289

10 ppb by USEPA and WHO [64, 65]. The basic principle of DLS is to measure nanoparticle size by

290

light scattering. AuNP can be aggregated by the target arsenic contaminant leading to significant size

291

difference than the individual AuNP, generating DSL signal linked to the arsenic concentration.

292

When sensitivity is not an issue, other challenges will be in place such as the means of trapping

293

arsenic

294

surface matrix with substances like glutathione (GST), dithiothreitol (DDT) and cysteine (Cys) [65,

295

66], given the high affinity of arsenic anion towards these matrices. For example, each arsenic anion

296

can bind with three DTT-conjugated AuNP through an As-S linkage resulting in change of color from

297

red to blue which can be conveniently monitored by DLS, UV-vis or naked eyes.

298

Similarly, lauryl sulphate[67] and ionic liquid[68] were also found to show high affinity towards

299

arsenic anion. Particularly, phosphonium ionic liquid has been used to specifically catch arsenite and

300

arsenate anions. The phosphonium’s strong interaction with arsenic species has been confirmed by

301

extended X-ray absorption fine structure. A protocol has been developed for visible test with a LOD

302

of 7.5 ppb, which is also well below the recommended 10 ppb by USEPA and WHO[68].

303

Selectivity of the detection by AuNP can be enhanced through the modification engaging enzyme

304

phosphatase (AcP) [69]. The enzyme catalyses the hydrolysis, which causes the appreciable

305

aggregation of AuNP with the eventual color change from red to blue. However, the arsenate is a

306

molecular analogue of AcP so that its presence can significantly interfere with the bioactivity of AcP

307

by slowing down the hydrolysis process. This may result in the reversal of color from blue to red.

308

Consequently. a LOD of 7.5 ppb was achieved for UV-vis test, or 27.5 ppb for visible test.

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284

AC C

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on to the AuNP surface and triggering the AuNP aggregate. An approach is to modify the

Page 21 of 44

ACCEPTED MANUSCRIPT Similarly, arsenic aptamer was also employed to modify the AuNP in order to increase the detection

310

selectivity [70, 71]. This aptamer can stabilise AuNP at a high concentration of NaCl solution and

311

retain the red color. When arsenic reacts with aptamer, individual AuNPs are aggregated with the

312

appearance of blue color. A LOD of 0.6 ppb was achieved for visual test whilst 0.77 ppb for

313

resonance scattering test.

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309

314 315

Figure 5. (a) Raman spectrum of As(III) and As (V)[63], (b)photo image of color change depending

316

on the concentration of As (III) (indicated) [65]. Reproduced with permission from [63] and [65].

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Table 4. AuNP-based AsO33- sensor. Signal/Detect Solution

Mechanism

Workin

or

complex 0.288-

Raman

Ref.

0.1 ppb

Tested in aqueous solution, in [61]

buffer pH 8.0 + 50 mM leads to AuNP aggregate, 23.04

air (after dry) and decreased

NaCl + 5.23 µM dye, 15 where dye’s Raman signal ppb

Raman signal upon AuNP

min incubation

aggregate, which are rarely

decreased.

M AN U

aptamer-dye

Ag-

25 nM aptamer + HEPES Aptamer-As

SC

Raman

Comments

g range

A drop of analyte was Adsorbed PVP on the NIL

TE D

AuNP-

LOD

RI PT

Matrix

nanocrystal

spread on the assembly substrate

assembly-PVP

surface and covered with facilitates the binding of

reported. 1.8 ppb

Patented approach

[62, 63]

surface

both arsenate and arsenite

EP

a cover slip

near the silver surface,

AuNP-GSHDTT-Cys-

DLS signal

AC C

318

allowing for enhancement of Raman.

PDCA added and 60 min Arsenic (III) show high 40 ppt - 10 ppt

Detection

incubation

anion interference unclear. But

affinity with DTT and 50 ppb

details

unclear, [65]

Page 23 of 44

ACCEPTED MANUSCRIPT

PDCA

leads to AuNP aggregate.

lowest LOD was reported so

RI PT

far AuNP-GSH-

Absorption

PDCA added and 120 min Arsenic (III) show high 2-20

DTT-Cys-

525 nm

incubation in the dark

affinity with DTT and ppb leads to AuNP aggregate.

AuNP-lauryl

Absorption

sulphate

at 730 nm /

coupling effect leads to ppb

530 nm

AuNP aggregate.

Absorption

liquid

at 630 nm / NH3●H2O (pH10.2), 30 between

Absorption

association 7.5-25

ionic

Absorption

unclear, [66]

anion interference unclear

Interference unclear

[67]

7.5 ppb

Ascorbic acid can reduce As [68] (V) to As (III) for their individual test. No interference

temperature.

of 10 mM PO43-

leads to AuNP aggregate

10 mM acetate buffer pH As (V) inhibits enzyme’s 7.5-75

AC C

37 °C, 5 min colorimetric the color from blue to red response.

AuNP-aptamer

details

min incubation at room phosphonium and As (III)

at 660 nm / 5.0, 80 min incubation at bioactivity and reversed ppb 520 nm

Detection

2 ppb

liquid ppb

EP

AuNP-enzyme

NH4NO3- Strong

inter-particle 5-500

TE D

AuNP-ionic

525 nm

mM

Arsenic’s

M AN U

pH 5.0 5 min incubation

SC

PDCA

30

2.5 ppb

10 mM MOPS buffer pH Aptamer-As

complex 1.26-

7.5 ppb

Groundwater

tested

with [69]

interference of Cu2+, F- and H2PO4-

1.26 ppb Water samples tested, cations [70]

Page 24 of 44

ACCEPTED MANUSCRIPT

at 650 nm / 7.0, incubation for 10 min leads to AuNP aggregate at room temperature, 60

interference unclear.

mM NaCl for aggregate AuNP-

Absorption

aptamer-

at 650 nm / 7.0, 30 min incubation at leads

surfactant

520 nm, or 25 °C, CTAB added for surfactant that aggregate

10 mM MOPS buffer, pH Aptamer-As

release

of ppb

SC

to

complex 1-1500

AuNPs

another 30 min incubation

Water samples tested, cations [71] interferences avoided, anion interference unclear.

AC C

EP

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319

0.6 ppb

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RS

interferences avoided, anion

RI PT

520 nm

200 ppb

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320

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5. AuNP-based sensors for PFOS / PFOA

323

PFASs feature unique physical and chemical properties, such as hydrophobicity and oleophobicity,

324

which have not being evidenced in other compounds. Due to these unique properties they have been

325

widely used in such applications as clothing, upholstery, carpeting, painting, food containers,

326

cookware, AFFF etc.[72]. On the other hand, PFASs also exhibit extreme stability with respect to

327

thermal, chemical and bio-degradation [73, 74], a property attributed to inert fluoro-carbon skeletons

328

making them extremely resistant to degradation under natural environmental conditions [75, 76].

329

Consequently, their continuous usage and inertness have led to their global distribution and

330

accumulation in the environment. This has in turn raised serious concerns about their impact on the

331

environment and public health [2, 77, 78]. Particularly, anionic fluorosurfactants of PFOS and PFOA

332

have been listed as emerging contaminants by USEPA and the recommended concentration in

333

drinking water is 70 ppt for individual or combined PFOS and PFOA [79, 80].

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322

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334

The binding event for anionic fluorosurfactants detection is based on the specific fluorine-fluorine

336

interaction [81-83]. For example, Takayose et al. developed a colorimetric detection of

337

perfluorooctanoic acid (PFOA) utilising thiol-terminated polystyrene-modified AuNP, as listed in

338

Table 5. They suggested the addition of PFOA in the AuNP matrix solution caused detachment of the

339

polystyrene layer from the surface of AuNP. Due to the strong interaction among PFOA molecules,

340

AuNP aggregated and the resultant colorimetric reaction was monitored [84].

AC C

341

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335

342

Another approach is the selective insert of the C-F skeleton into the self-assembled monolayer (SAM)

343

on the AuNP surface. Niu et al. have developed a sensitive colorimetric detection of fluorosurfactants

344

using poly (ethylene glycol) and perfluorinated thiols modified AuNP [85]. In this mixture of SAM,

345

the former provides the stability of the surface-functionalised AuNP whist the latter offers the

346

selectivity for the detection of fluorosurfactants, as indicated in Figure 6. After a 30 minute incubation

Page 27 of 44

ACCEPTED MANUSCRIPT 347

in the presence of fluorosurfactants including PFOS, PFOA and others (perfluoroalkyl chain >6), a

348

limit of detection of 10 ppb was achieved for all the fluorosurfactants.

349 350

Rather than depending on the F-chemistry to provide the selectivity towards the fluorosurfactants’

351

detection, a bioassay method was

352

including PFOS can bind to the peroxisomal proliferator-activated receptor-alpha (PPARα), which

353

forms heterodimers with retinoid X receptors (RXRs) and binds to PPAR responsive elements. The

354

detection limit was reported to be as low as 2.5 ppt. Although they still employed the AuNP as the

355

platform, subsequent silver enhancement is necessary to reach a limit of detection of 10 pM (~5 ppt

356

for PFOA). Though the limit of detection is excellent in this case, the time-consuming sample

357

preparation and requirement for overnight incubation at 4 °C are the major drawbacks in this approach.

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developed by Xu and Shu et al.[86, 87]. Fluorosurfactants

358

As a sensor platform, AuNP has witnessed significant development recently. However, other

360

nanomaterials might exhibit some advantages too. For example, in terms of SERS enhancement,

361

silver is much better than gold [19, 88]. Consequently, silver nanoparticles (AgNP) have been tried

362

for SERS detection of fluorosurfactants [89]. Graphene oxide was used to increase the loading affinity

363

of fluorosurfactants considering their hydrophobic and oleophobic properties. The introduction of dye

364

selectively precipitated anionic surfactants by forming an ion-pair, which provided the selectivity.

365

Consequently, a limit of detection of 50 ppb for PFOA was achieved from a ground water sample.

EP

AC C

366

TE D

359

367

Another example is the combination of a nanosheet of carbon nitride (C3N4) with molecularly

368

imprinted polymer (MIP) and electrochemiluminescence (ECL) for the sensitive detection of PFOA

369

with a detection limit of 10 ppt [90]. The illumination detection using a photo multiplier tube might

370

limit the on-site application. Similarly, QD, rather than AuNP, has also been developed for the

371

detection of PFOA with a limit of 300 nM (~120 ppb) when fluorescent detection was employed [91].

Page 28 of 44

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ACCEPTED MANUSCRIPT

372

Figure 6. Schematic drawing of the response toward to fluorosurfactants (a) and absorbance response

374

in the presence of 500 ppb of each non-fluorinated compounds (b). Reproduced with the permission

375

from [85].

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Table 5. AuNP-based PFASs sensor. Mechanism

Working

or Polymer-

Naked eye

LOD

Comments

~100 ppm

PFOA,

RI PT

Signal/Detect Solution

Ref.

range 30 h incubation

PFOA

AuNP

detached NIL

polystyrene

from

M AN U

AuNP surface and lead

SC

Matrix

qualitative [84]

test, no real sample (~250 µM)

to AuNP aggregate. SAM-AuNP

Absorption

Water sample was pre- F-chemistry

induced 10-1000

naked eye

mL methanol. Test: 0.2 mL concentrated

sample

is

solution,

with

30

ppb

[85]

fluorosurfactants (perfluoroalkyl chain > 6), Coloration not obvious

EP

mixed with 0.8 mL AuNP

10 ppb

TE D

at 520 nm, treated from 200 mL to 5 AuNP aggregate

min

AC C

377

incubation. Bio-AuNP

Optical

Water sample was pre- Bioassay

based

on 100 pM -1 10 pM

PFOS,

PFOA

etc. [86]

tested. Bio-process is

density after treated from 1000 mL to 1 silver enhancement of µM (~5 ppt) silver

mL methanol. Test: several AuNP and interaction

time-consuming and

Page 30 of 44

ACCEPTED MANUSCRIPT

enhancement

steps, 30+ min incubation

among

ligands,

needs

trained

(~50 ppt operator.

PPARα and PPRE

QD-bioassay

Fluorescence

Water sample was pre- Bioassay

based

with

M AN U

400 nm

1 mL QD + water sample + PFOA lead to QD 0.5-40µM

with

mL

Na2HPO4-NaOH aggregate

pH10, 10 min incubation

excitation at 365 nm electrochemil In uminescence

0.1

M

SERS

(~200-

tested. [87]

PFOS Bioassay

is

time-

consuming and needs operator.

0.3 µM

PFOA

spiked

in [91]

textile sample tested (~120 ppb)

florescence quenching

(pH6) Co-reactant

16000 ppb)

S2O82- 0.02-400

0.01 ppb

PFOA

spiked [90]

containing 3 mM Na2S2O8 oxidised PFOA and ppb

environmental

and 0.1 M Na2SO4, 20 min lead to decreased ECL

sample test.

incubation. AgNP-dye-

PBS

AC C

MIP-C3N4

2.5 ppt

and

TE D

at 560 nm 1

EP

Fluorescence

to

mL methanol. Test: several PPARα.

excitation at steps, 120 min incubation

SAM -QD

binding

on 2.5-7.5 ppt

SC

at 605 nm treated from 1000 mL to 1 PFOS’

RI PT

500 ppb)

signal

10 mM NaCl (pH5.8), 120+ Dye

co-participate NIL

50 ppb

Spiked

PFOA

in [89]

Page 31 of 44

ACCEPTED MANUSCRIPT

min incubation.

oxide

PFOS/PFOA/6:2FTS

groundwater sample

onto

tested.

AgNP-graphene

AC C

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oxide surface

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graphene

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ACCEPTED MANUSCRIPT 378

379

6. Performance concerns

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380

In this review, AuNP based colorimetric detection, fluorescent detection and SERS detection have

382

been discussed. However, with the continuous advent of newer technologies more opportunities are

383

being explored through other techniques as well. For example, recently, a method based on the

384

dynamic photo scattering (DPS) was established with comparable sensitivity with SERS [23, 24].

385

However, the setup is much simpler and cheaper than Raman in terms of sensitivity and thus has

386

attracted various researchers.

387

For sensing applications the traditional methods in professional laboratories are being challenged by

388

portable sensor, kits and devices due to demands for on-site analysis. The latter promises faster

389

screening approaches coupled with effective determination of contaminants.. However these kits lack

390

certain characteristics and have limitations, and are yet to be proven as good as laboratory sensing

391

methods.

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6.1. Selectivity vs. sensitivity

394

There are two kinds of requirements for the improvement of sensors, which are selectivity and

395

sensitivity [23, 24]. The selectivity usually originates from the surface functionalisation of AuNP

396

whilst the sensitivity depends on multiple parameters such as the binding event, the convertion

397

efficiency to optical signal, and the background noise. Considering fluorosurfactant as an example,

398

the selectivity mainly stems from the electro-static interaction and the hydrophobic/oleophobic

399

fluorine-carbon chains [8, 10, 89]. Thus due to the “fluorous nature” of the fluorocarbon skeleton,

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ACCEPTED MANUSCRIPT 400

they exhibit both hydrophobic and oleophobic properties, which are not evidenced from other

401

materials [81-83]. Consequently, they can be selectively extracted and detected.

402

However, the interference from other co-existing ions will have a serious impact given that the

404

concentration of fluorosurfactants is commonly at the level of ppb (< 10 nM) whilst other anions are

405

present at a much higher level, such as Cl- (µM – mM). It is thus recommended that a pre-treatment of

406

samples should be included, which must be carried out prior to the application of AuNP sensing, for

407

improved selectivity and sensitivity.

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6.2. Sample-preparation

410

A highly sensitive test usually warrants a better sample preparation. In a modern analytical chemical

411

lab the sample preparation generally consumes 50 - 90% of the time duration and labour cost [92],

412

which requires professionally trained lab personnel to perform the sample preparation processes.

413

Considering the complexity of environmental samples whose matrices comprise several physical

414

matters, wider biotas and multiple types of chemicals, extensive sample pre-cleaning and pre-

415

concentration are always required as important pre-requisites and most importantly are unavoidable

416

too.

EP

AC C

417

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409

418

Particularly when the target analyte’s concentration is much lower than that of the co-existing

419

species/ions, this will pose a challenge for an accurate analysis which is the case for fluorosurfactant

420

detection, as discussed in Section 4. Fortunately, recent developments on the new testing platform

421

might offer an opportunity to integrate the sample preparation and sample analysis in a single step. A

422

field portable testing kit provides this advantage so that there are no additional or extra steps to carry

423

out the sample preparation [33, 93]. This is critical for rapid screening procedures targeting a Page 34 of 44

ACCEPTED MANUSCRIPT 424

particular group of analytes irrespective of whether a qualitative or quantitative approach is taken by

425

employing on-site test kits.

426

6.3. Quantitative vs. qualitative

428

Visual colorimetric detection is dependent on the assessment of color changes, which means it is a

429

semi-quantitative test at best, with the color chart provided as reference. For color reading,

430

interference from the background should be taken into account. The illumination on a sunny day

431

differs from that on a cloudy or rainy day. For accurate reading, a reading kit is recommended.

432

Alternatively, a background correction can be applied using software [94]

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427

433

Mobile phones are now near-ubiquitous; as of 2016 apple sell > 1 billion iPhones [95]. Modern

435

mobile phones, particularly high tech smartphones, are equipped with valuable and relevant

436

capabilities including high resolution camera, high speed processor, touch screen display, higher

437

memory capacity, long life battery, etc. Therefore, smartphones offer a unique approach for increasing

438

device availability and accessibility [96]. Thus a smartphone-based testing platform could drive a new

439

direction in sensor development for on-site screening applications [33, 94, 96]. In addition, it is

440

anticipated that with the increased development of user friendly smartphone apps in combination with

441

other technologies, such as GPS to mark the test position, on-line help and demonstrations to assist

442

operation by non-techniciansetc., this will provide an opportunity and impetus towards the

443

development of more efficient and faster portable sensor kits.

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ACCEPTED MANUSCRIPT 6.4. Testing kit vs. portable device

446

Over recent decades, various testing kits have appeared on the market [97, 98]. Most of them work via

447

a similar operational procedure with a pH testing paper, such as a pregnancy test stick. In order to

448

increase the selectivity and sensitivity, paper-based devices have also been developed [99]. Their

449

working principle is usually based on micro-fluidics technology to pre-concentrate or pre-extract the

450

target analyte. However, most of them are still capable of only performing semi-quantitative tests,

451

such as ”astkCARE”[100].

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445

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452

In contrast, portable devices which are integrated instruments can be easily carried and handled at on-

454

site testing areas with much ease [93, 98, 101, 102]. The weight for any such handheld device is slated

455

to be at < 10 Kg [103] and the analytical chemistry is working in the background to support the

456

process for a quantitative test [11]. Once the chemistry behind the analysis is integrated well within

457

these devices with an appropriate app, it might become the preferred system even for non-trained

458

personnel, in the near future.

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459

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6.5. Kinetics vs. thermodynamics

461

For color reading, kinetics should be distinguished from thermodynamics, particularly when the

462

colorimetric reaction of AuNP is dominated by a kinetic process. This situation might be more serious

463

when the binding or recognition process is slow, such as the CN- etching of AuNP, which usually

464

takes ~30 min for incubation, as listed in Table 3. However, the monitoring of the kinetics might

465

provide additional information for characterisation, particular when the monitoring process can be

466

automated, such as through a smartphone when it is employed as the detector.

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467

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ACCEPTED MANUSCRIPT 6.6. AuNC, other nanomaterials

469

While AuNP is a common platform for colorimetric detection, other shape/types of nanomaterials

470

should also be paid attention [16, 17] [18]. For example, the Au nanocluster features a strong

471

fluorescence which can be used as fluorophore, as discussed in Tables 2 & 3. An Au nanoflower,

472

nanorod, or nanowire might exhibit strong enhancement on surface plasmon, which is helpful for

473

SERS detection [20-22]. Other platforms including silica nanoparticle, core-shell nano composite [51],

474

bi-metal nanoparticle [55]and MIP [90] are also reported for colorimetric sensing.

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475

7. Perspectives

477

In general, it is expected that both the testing kit and portable device will receive more and more

478

attention for rapid and random on-site assessments. AuNP-based sensors in particular along with

479

other nano-sensors will continue to be employed to develop these systems, which exhibit the capacity

480

for a sensitive and selective test on a wide range of targeted anions of environmental concern.

481

Appropriate analytical protocols and regulatory governance will aid in improving the operating

482

capabilities of these systems.

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AUTHOR INFORMATION

485

Corresponding Author

486

Phone: +61 2 4913 8740; Fax: +61 2 4913 8740; Email: [email protected]

487

Notes

488

The authors declare no competing financial interests.

489

ACKNOWLEDGMENT

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The authors kindly acknowledge funding support provided by CRC CARE.

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ACCEPTED MANUSCRIPT 491

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1. Glod nanoparticle-based optical sensors are critically reviewed. 2. The selected targets include a controlled anion of F-, toxic contaminants CN-, AsO33-/AsO43-,

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