AYUNTAMIENTO RÍO BRAVO, TAM

Compared to metal films deposited onto a roughened electrode either electrochemically or in vacuum, there are several advantages in using nanoparticles (NPs) as SERS active substrates. Because the dimensions of the surface morphologies produced via electrochemical roughening are crucial to the observation of SERS,99 it is often difficult to generate such „roughness‟ uniformly across a macroscopic surface leading to in homogeneity in the intensities of SERS bands (one often refers to the presence of hotspots giving rise to exceptional SERS activity in localised regions of a sample). However, if all NPs are of equal diameter and of optimal size to generate a maximal SERS response, simply coating a substrate with such NPs will immediately render it SERS active with uniform homogeneity across the whole electrode surface. Also, using NPs in this way does not require the use of expensive vacuum evaporation chambers to produce a SERS active film and so reduces cost and makes such substrates available to all.

Usually, nanoparticles are created with sizes between 2 and 100 nm since SERS phenomena only happen on metal surfaces containing surface morphological features lying within these ranges. In particular, metal NPs with sizes between 10 and 100 nm possess special surface plasmon resonance properties which results in great enhancement of the Raman signal for molecules adsorbed on (or close to) their surfaces.100, 101 Hence, the design and preparation of metal nanoparticle substrates has become the main focus in order to achieve strong signal intensity and reproducibility. Metal NP colloids are the most commonly used SERS substrate because their manufacture usually involves simple experimental steps such as mixing of liquid solutions, heating and separation. Another great advantage of NPs is that they can be used as catalytic components of technical catalysts. Colloidal metal nanoparticles have been synthesised with controlled sizes and shapes to meet various requirements in catalysis researches.102-105 Here, the preparation of spherical metal NPs, aggregation of metal NPs, bimetallic NPs and metal NPs with other kinds of shapes will be discussed. Their influence on the SERS signals in terms of the size and shape and the excitation wavelength will be noted as well.

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Wet chemistry methods have been developed mainly for the preparation of metal NP colloids, such as chemical reduction, laser ablation and photoreduction. The development of the shape of the NPs is no longer limited to spheres but rather, with the addition of suitable modifiers NPs of various shapes and size distributions have emerged.103, 105 Moreover, the composition of the NPs may now range from a single component to multiple components (or alloys). No matter what kind of shape or composition, the aggregation step during the preparation process is regarded as a key stage with respect to obtaining better SERS signals by presenting „hot spots‟ to the probe laser beam of uniform intensity and distribution.106

Gradually, new types of metal NPs have been designed according to the fact that the SERS intensity depends on the excitation wavelength and the strength of the plasmons propagating on the surface of the metal NPs. These kinds of core-shell metal NPs and dimers of NPs can generate enhancement factors as high as 106.107 This exquisite control of particle shape, size and composition both for tailoring the SERS activity and the catalytic activity/selectivity of NPs has brought enormous attention from scientists worldwide.

Colloidal spherical particles are the oldest class of NPs reported. In 1979, Creighton used ice- cold NaBH4 solution to reduce AgNO3 and KAuCl4 to obtain silver and gold NPs respectively. He chose pyridine as the probe to investigate the SERS activity of these metal NPs and discovered a strong dependence on the excitation wavelength.108 Typical methods of preparing these kinds of Ag NPs involved reduction of AgNO3 by sodium citrate under reflux.109, 110 The size of the Ag NPs was usually between 60 and 80 nm. These are also the simplest and most common methods of synthesising spherical metal NPs. The reducing agents include sodium citrate111, sodium borohydride112, hydrazine113 and hydroxylamine hydrochloride114, 115. Then by adjusting the parameters such as the reaction temperature, pH of the solution and the kind of metal salt, the size and the aggregation state of metal NPs would be controlled. For instance, a very stable silver colloid exhibiting a particle size between 40-70 nm was prepared by Nickel et al.116 by reducing aqueous silver nitrate with hydrazine dihydrochloride in a weakly alkaline solution. These Ag NPs were reported to show a high SERS signal with the dye molecule Nile Blue A. Also, Li et al. prepared 17 nm in diameter spherical Ag NPs with mercaptoacetic acid as capping agent and noted that the SERS signal was determined by the particle sizes and degree of aggregation.117 The preparation of gold NPs was developed by Frens118, who used a similar method to reduce chloroauric acid using sodium citrate. The particle sizes produced were in the range of 20- 100 nm. Peter et al. prepared Au NPs based on the seed-mediated growth method in which

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HAuCl4 was reduced with sodium acrylate and subsequently refluxed for 30 min.119 Schwartzberg and his co-workers utilized the template method to manufacture hollow, spherical Au NPs and obtained strong SERS signals from 4-mercaptobenzoic acid adsorbed on the NP surfaces due to the homogeneous structure.120 Besides the commonly used synthesis of NPs in aqueous solutions, NP preparation involving organic solvents was invented by Shen et al.121 In this report, the monodisperse noble-metal NPs with SERS effect were synthesised in a direct reaction of the metal salt with oleylamine in toluene.

Apart from the wet chemistry methods mentioned above, laser ablation and photoreduction are also the most used methods to prepare SERS active NPs. The big advantage for the NPs prepared by these methods, which are considered as „chemically pure‟, is that they avoid the influence of residual ions on the SERS signal during the chemical reaction. The typical procedure for laser ablation includes using a pulse laser (1064 nm from a Nd:YAG laser) to ablate metal plates or foils in distilled water or a NaCl solution.122, 123 Reproducible SERS spectra can be obtained from the metal colloids with optimised size distribution after taking the parameters, such as laser pulse energy, the time of ablation and the laser beam focussing into account. Meanwhile, the general procedure for the photoreduction method involves using light (gamma or UV laser irradiation) to reduce the metal salt.124-127 For instance, Ahern et al. observed the SERS signals of pyridine and biotin using Ag colloids prepared by laser irradiation.128

As mentioned, there is a strong relationship between the size of the metal NPs and their SERS activity. It has been proved that the variation of the localised surface plasmon with NP size plays an important role in affecting the SERS activity.129-133 Jang et al. discovered that the SERS enhancement of 4-biphenylmethanethiolate on Au NPs was very weak for the size of 11 nm and much better for larger particles (43 and 97 nm).134 On the other hand, Nie et al. explored the relationship between the optical excitation wavelength and particle size by studying the SERS activity on different sizes of Ag NPs with different laser lines (488, 568 and 647 nm).78 Figure 1.5 clearly shows that the SERS enhancement obtained from the NPs is strongly dependant on the size of the particles and the excitation wavelength. Therefore, in order to obtain the best SERS signal, one needs to decide the size of the NPs and the preferential wavelength to excite surface plasmon resonance on them.

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Figure 1.5 SER spectra of spatially isolated single Ag NPs, which were selected by wide- field screening for maximum enhancement at (a) 488 nm, (b) 567 nm and (c) 647 nm. Reprinted from reference 78.

The stability of the metal NPs colloidal system is another important factor to be considered for various applications. Those prepared by the traditional citrate reducing methods are known to be very stable over a long time. However, the chemical properties of the surface will change remarkably after several days, resulting in loss of consistency and intensity of the SERS signal. Hence, addition of stabilizers, such as poly(vinyl alcohol), poly(vinylpyrrolidone) and sodium dodecyl sulphate, to prevent aggregation was utilised and so improve the stability of the colloids.135-137 Nevertheless, the disadvantages of the stabilizer would include generating interfering signals as a consequence of the added polymer together with increasing the electrostatic barrier, resulting in a reduction of the ability of analytes to adsorb. Hence, to avoid using polymers that are difficult to remove post synthesis, alternative methods of stabilising NPs have been developed including silica or bovine serum albumin (BSA) coatings.138, 139

In order to obtain much stronger SERS signals, aggregation of the metal colloids is an essential step. It has been indicated experimentally and theoretically that, when single NPs form aggregates of two or more NPs, a stronger enhancement can be generated due to the coupling of the electromagnetic field around the vacant interstices separating the particles.76, 140-144

Therefore, to facilitate aggregation, hydrosol activation is induced by adding inorganic salts, surfactants, organic amines or mineral acids to the colloids.145-147 For example, the Ag

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colloids with evident SERS activity prepared by reduction with hydrazine and aggregation of the NPs by Cl- was reported by Nickel and his co-workers.113It was found that Cl- can improve the enhancement greatly because of the increase of electromagnetic field induced by the anion and the reorientation of the molecules on the NPs. Also, the analyte itself was reported to cause aggregation of the NPs and show different enhancements by Heard et al.136 They found that the addition of cetylpyridinium and cetylquinolinium salts changed the colour of the solution due to the aggregation of the metal NPs into clusters. This aggregation, which produces strong enhancement of Raman signal, proved that it plays an important role for the observation of enhanced Raman scattering from an adsorbed species.

Besides silver or gold NPs with a single composition, bimetallic or core-shell bimetal NPs which combine the SERS activities of both metals and generate greater enhancement have been developed.148-150 By changing the ratio between the two metals or the shell thickness of these NPs, it will cause a shift of the resonance frequency. Fang et al. first attempted to simply mix the prepared Ag and Au NP colloids and explored SERS of them coated with dye.148 They found that a certain ratio of mixed Ag and Au colloids induces a specific aggregation and results in stupendous increase in the SERS activity. However, in order to achieve better SERS enhancement with the proper control of the metal NPs, the most common method for making bimetallic NPs is using a chemical reaction as reported in the literature.149-152 Later, core-shell NPs with the composite either Au@Ag or Ag@Au were also fabricated and utilised for single molecule detection.153, 154 Recently, Tian et al. proposed the „borrowing SERS enhancement‟ concept and chemically synthesised Au NPs coated with ultra-thin shells of different transition metals. By the long-range effect of the enhanced electromagnetic field created by the Au core, the non SERS active transition metals remarkably improve their Raman effect.106 It has been widely used since its invention for the study of various catalytic reactions on the transition metal surfaces.