Gold Triangular Nanoprisms and Nanodecahedra: Synthesis and

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GOLD TRIANGULAR NANOPRISMS AND NANODECAHEDRA – SYNTHESIS AND INTERACTION STUDIES WITH LUMINOL TOWARDS BIOSENSOR APPLICATIONS Selvaraj Naveenraj, Ramalinga Viswanathan Mangalaraja, Jerry J. Wu, Abdullah M. Asiri, and Sambandam Anandan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02976 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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GOLD TRIANGULAR NANOPRISMS AND NANODECAHEDRA – SYNTHESIS AND INTERACTION STUDIES WITH LUMINOL TOWARDS BIOSENSOR APPLICATIONS Selvaraj Naveenraja,b, Ramalinga Viswanathan Mangalarajab*, Jerry J. Wuc, Abdullah M. Asirid, Sambandam Anandana** a. Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India. b. Advanced Ceramics and Nanotechnology Laboratory, Department of Materials Engineering, University of Concepcion, Concepcion, Chile. c. Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan. d. The Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21413, P.O. Box 80203, Saudi Arabia. KEYWORDS Gold Nanoparticles, Luminol, Triangular Nanoprisms, Nanodecahedra, Static Quenching, Interaction Studies

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ABSTRACT

Gold triangular nanoprisms and nanodecahedra (pentagonal bipyramids) were synthesized in the absence and presence of nanoseeds by a simple solvothermal synthesis through the reduction of Auric Chloride (HAuCl4) with poly(vinylpyrrolidone) (PVP) in N,N-dimethylformamide (DMF), respectively. These gold nanoparticles exhibit two plasmon resonance bands. The interaction of these gold nanoparticles with luminol was investigated using UV-vis and fluorescence spectroscopy since hefty number of environmental and biological sensors are based on the combination of luminol and gold nanoparticles. The gold nanoparticles quenches the fluorescence of luminol through static quenching mechanism i.e., ground state complex formation which was confirmed by both the absorption spectroscopy as well as the time-resolved fluorescence spectroscopy. The Stern-Volmer quenching constant and the effective quenching constant determine that gold nanodecahedra has more interaction with luminol than that of triangular gold nanoprisms. The distance between the gold nanoparticles and luminol, calculated using FRET theory, is less than 8 nm which indicates the efficient energy transfer during interaction. These results are expected to be useful for the development of novel sensors.

1. Introduction Metal nanoparticles (MNPs) are widely used as the functional materials in physics, chemistry, biology and medicine since MNPs exhibit fascinating unique physical, chemical, and magnetic properties which can be effortlessly tuned just by tailoring their size and shape to fit a particular application ranging from biological sensing to information storage 1. Gold nanoparticles (GNPS), being the most stable MNPs, have been expected to serve as the key materials and building blocks for innumerous variety of nanodevices in this century owing to their critical size-related


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unique surface chemical, optical, photonic, electrical and catalytic properties as well as their facile preparation methods with controlled size and shape 2. Even though Michael Faraday reported the first scientific literature of GNPS in the 1850s 3, the research publications about GNPs started exponentially increasing only in the end of the twentieth century owing to their widespread prospective applications 2a, 2c. Several papers have been published with the combination of GNPs and luminol especially for their environmental and biological sensing applications using the chemiluminescence (CL) and electrogenerated chemiluminescence (ECL). Ultrasensitive sensors can be constructed using the ECL of GNPs-catalyzed luminol for the detection of glucose 4, cholesterol 5, and antiaging agent glutathione 6. By means of the CL of GNPs-catalyzed luminol, aminothiols can be detected even in the presence of other essential amino acids and biomolecules 7. In addition, it can be utilized in the detection and determination of several bioactive substances such as estrogens 8, neurotransmitter catecholamines, and enzyme inhibitor Captopril 9. It can be even used in the detectors for high-performance liquid chromatography

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. The CL of luminol functionalized

GNPs can be utilized not only in the construction of a turn-on chemiluminescence aptasensor for the detection of interferon-gamma

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, but also in the construction of ultrasensitive

chemiluminescence-based sensors for the detection of bioactive substances such as glucose vitamin C

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and antibacterial agent minocycline

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12

,

. Luminol-labeled GNPs can be used to

analyse the highly diluted blood samples containing intact or lysed red blood cells using its ultrasensitive CL

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. ECL of Luminol functionalized GNPs can be used in the construction of

sensors for the industrial dye malachite green 16, antibiotic chloramphenicol 16, DNA assay 17, the protease thrombin and mucin-1

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. It can be used in the detection of cancer biomarkers such as telomerase

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. These vast applications of the GNPs-luminol combination create an interest to


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understand the interaction of GNPs with luminol. This molecular interaction can be studied using the optical spectroscopic techniques rather than the conventional approaches due to their sensitiveness, usage of low concentration, abundance of qualitative and quantitative information and relatively easy 21. In this regard, GNPs were synthesized by solvothermal method using DMF as the solvent in the absence and presence of gold nanoseeds due to the shape controlling ability of the nanoseeds 22

. Thus formed nanoparticles were characterized using TEM and SAED patterns. Their optical

properties were studied using UV-vis spectroscopy. Then their interaction with luminol was studied using multispectroscopic techniques such as UV-vis spectroscopy, steady-state fluorescence spectroscopy, and time-resolved fluorescence spectroscopy under physiological conditions. The binding information, including quenching mechanisms, binding parameters, binding modes, and the distance were investigated. 2. Experimental 2.1. Materials Auric Chloride (HAuCl4.3H2O) was of analytical grade purchased from Aldrich chemicals and used as received. All solvents were of extra pure analytical grade and were used without any further purification. 2.2. Methods 2.2.1. Solvothermal synthesis of GNPs in the presence and absence of gold nanoseeds


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The freshly prepared aqueous solution of HAuCl4 (1 ml of 0.02 M) was added into the singleneck round bottomed flask with poly(vinylpyrrolidone) (PVP) (1 g) in a 100 ml mixture of N,Ndimethylformamide (DMF) and H2O (19:1) at room temperature. After being mixed thoroughly, gold nanoseeds (1.2 ml) of size 3-5 nm (Figure S1, supplementary information) synthesized by following Teranishi et al.,

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procedure were added to the resulting colourless solution and then

stirred continuously at room temperature (30 °C) for 15 min. Then this colourless solution was turned to pale blue colour upon refluxed at 156 °C for 2 hours by placing the single-neck round bottomed flask on a temperature controlled oil bath equipped with a cooler to keep the reflux. Without adding gold nanoseeds, the same procedure was followed however a pale orange colour solution results. Thus prepared gold nanoparticles were further utilized for characterization. 2.2.2. Interaction of GNPs with luminol 4 µM luminol solution was prepared in DMF. 1.33 µM of the prepared GNPs solution was added to 3ml of 4 µM luminol solution. The effect of GNPs on luminol was monitored using UVVisible spectroscopy, and fluorescence spectroscopy. 2.3. Analytical procedures The surface morphology and particle size of the synthesized nanoparticles were analyzed using TEM (TECNAI G2 model). Samples were coated on copper grid at normal atmospheric temperature and pressure. Ultraviolet-Visible absorption spectra were recorded on a T90+ UV/Visible spectrophotometer in the range of 200-800 nm at a scan rate of 250nm/min with an interval of 1 nm. All spectra were collected against the background spectrum of solvents. Fluorescence spectra were recorded on SHIMADZU Spectrofluorophotometer at a scan rate of 500nm/min in the emission wavelength range of 330- 900 nm exited at 300 nm. Fluorescence


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lifetime measurements were carried out in a picosecond time correlated single photon counting (TCSPC) spectrometer with tunable Ti-sapphire laser (TSUNAMI, Spectra physics, USA) as the excitation source. The fluorescence decay curves were analyzed using the software provided by IBH (DAS-6). 3. Results and discussion 3.1. Solvothermal synthesis of GNPs using gold nanoseeds GNPs were synthesized by solvothermal synthesis since it is a very simple technique having several advantages such as cost effective, short processing time, controlled morphology, environmentally benign, and relatively low temperature synthesis

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. In this solvothermal

method, DMF was not only acted as the solvent but also play the role of the reducing agent in the formation of GNPs by reducing [AuCl4]- ion 25. DMF reduces AuCl4- to Au0 as follows:

In the presence of nanoseeds, PVP in DMF also controls the reduction effect of [AuCl4]- ion 26. The TEM images of the solvothermally synthesized GNPs synthesized in the absence and presence of nanoseeds were shown in Figure 1. The TEM image suggested that triangular gold nanoprisms with edge length of 80 nm were formed in the solvothermal method using DMF as solvent whereas in the presence of nanoseeds, uniform gold nanodecahedral (pentagonal bipyramid) particles with pentagon side length of 65 nm, inradius of 45 nm and height of 75 nm were formed 27. Their corresponding SAED patterns (inset in figure 1) suggested that triangular gold nanoprisms were polycrystalline in nature whereas the gold nanodecahedra were monocrystalline in nature

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. This confirms that the nanoseeds altered the morphology and


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crystallinity of the formed GNPs in the solvothermal synthesis which can be attributed to the catalytic nature of the complex formed between Au seeds and DMF 25a.

a

b

65 nm

75 nm

Figure 1. TEM images of solvothermally synthesized gold nanoparticles in the absence (a) and presence (b) of nanoseeds. Inset is their corresponding SAED pattern. UV–vis absorption spectroscopy is the most widely used method for characterizing the optical properties of the nanoparticles. The UV-vis spectra of triangular gold nanoprisms and gold


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nanodecahedra, shown in figure 2, displayed two surface plasmon resonance bands. The triangular gold nanoprisms showed two distinct bands, an in-plane quadrupole plasmon resonance band at 723 nm and an in-plane dipole plasmon resonance band of pseudosperical nanoparticles that formed concomitantly at 560 nm

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. The gold nanodecahedra showed two

dipolar plasmon bands at 614 and 660 nm which arise from the polar and the azimuthal modes of the decahedral nanoparticles

23b, 25a, 27

. The absence of the quadrupolar plasmon band confirms

the size controlling nature of the nanoseeds in the formation of gold nanodecahedra having a pentagonal side length of 65 nm 27, 30.

A B

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0.4

0.3

300

400

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600

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Wavelength (nm)

Figure 2. The absorption spectra of triangular gold nanoprism (A) and gold nanodecahedra (B). 3.2. Effect of GNPs on the fluorescence spectrum of Luminol Fluorescence measurements were performed to investigate the interaction of luminol with GNPs. Since the GNPs has absorption around 300 nm, the fluorescence intensities were corrected for


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inner filter effects i.e., the absorption of exciting light and reabsorption of emitted light using the following relationship 31:

Fluorescence Intensity

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F

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F

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Figure 3. Effect of GNPs [(a) Triangular nanoprism and (b) nanodecahedra] on the fluorescence spectra of luminol (4 µM). From A-F curve, GNPs concentrations are 0, 1.33, 2.67, 4, 5.33, and 6.67 µM.


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= × ⁄ where Fcor and Fobs are the corrected and observed fluorescence intensities, respectively, Aex and Aem are the absorption of the system at excitation and emission wavelength, respectively. Figure 3 shows the fluorescence spectra of luminol in the absence and presence of GNPs after corrected inner-filter effect. It can be seen that luminol had a strong fluorescence emission band at 409 nm when excited at 300 nm. While increasing the concentration of GNPs, the fluorescence intensity of luminol decreased regularly without any shift in the emission wavelength maxima which indicated the occurrence of the interaction between luminol and GNPs through the formation of non-fluorescent ground state complex. The normalized quenching effects of GNPs on the fluorescence of luminol (Figure S2, supplementary information) suggested that at 1:1 molar ratio of GNPs and luminol, triangular gold nanoprism quenches about 31.4% of luminol’s fluorescence whereas gold nanodecahedra quenches about 34.5% of luminol’s fluorescence. This observation indicates that the fluorescence of luminol was better quenched by gold nanodecahedra than that of triangular gold nanoprism.

The fluorescence quenching is usually analysed by the well-known and most habitually used Stern-Volmer equation 31c which is as follows: = 1 + = 1 + where F0 and F denotes the steady state fluorescence intensities in the absence and presence of the quencher Q respectively, Kq is the quenching rate constant of bimolecule, τ0 is the average lifetime of molecule without quencher (10-8 s), [Q] is the quencher concentration, KSV is the


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Stern-Volmer quenching constant. The obtained linear Stern-Volmer plots for the interaction of GNPs with luminol (Figure S3, supplementary information) indicate that the interaction follows only one kind of quenching mechanism i.e., either static or dynamic mechanism. From the slopes of the linear plots, the Stern-Volmer quenching constants (KSV) were calculated as 1.15×105 L.mol-1 for triangular gold nanoprism and 1.34×105 L.mol-1 for gold nanodecahedra. The bimolecular quenching rate constants, kq, of triangular gold nanoprism and gold nanodecahedra are found to be 1.15×1013 L.mol-1s-1 and 1.34×1013 L.mol-1s-1, respectively, which are greater than that of scatter procedure (2×1010 L.mol-1s-1)

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. This shows that the quenching mentioned

above is not initiated by dynamic collision but from the ground state complex formation or static quenching. 3.3. Quenching mechanism The fluorescence quenching can be usually classified as either dynamic quenching or static quenching

21b, 33

, which depends on the occurrence of the collisional encounters or the ground-

state complex formation between the fluorophores and quenchers, respectively. Absorbance measurements were performed to investigate the interaction between luminol and GNPs since it is one of the effective method to distinguish static and dynamic quenching 31a, 33. The absorption spectrum of luminol (Figure 4) exhibits two peaks in the ultraviolet region at 296 nm and 355 nm which correspond to the S0 (π) →S2 (π*) transition and S0 (π)→S1 (π*) transition respectively 34. On the addition of GNPs, the absorbance around 296 nm increases with a blue shift in the wavelength maxima which suggest the formation of ground state complex with a decrease in hydrophobicity and confirms the static quenching mechanism since dynamic quenching does not


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affect the absorption spectra 31a, 33. There is not much increase in the absorbance around 355 nm compared to that of around 296 nm.

0.8

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0.6

F 0.4

0.2

0.0 300

325

350

375

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Wavelength (nm)

Figure 4. Effect of GNPs [(a) Triangular nanoprism and (b) nanodecahedra] on the absorption spectra of luminol (4 μM). From A-F curve, GNPs concentrations are 0, 1.33, 2.67, 4, 5.33, and 6.67 μM.


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a

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Luminol Luminol + GNPs

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2

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Figure 5. Effect of GNPs [(a) Triangular nanoprism and (b) nanodecahedra] on the fluorescence life-time spectra of luminol (4 μM). From A-F curve, GNPs concentrations are 0 and 6.67 μM. Dynamic and static quenching can also be distinguished by their difference depending on life time measurements value 21a, 32. The fluorescence lifetime of luminol shows a double exponential decay. With the addition of both the GNPs, there is no considerable change in the lifetime of


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luminol (Figure 5). This observation suggests the formation of ground state complex between luminol and GNPs which confirms the static quenching mechanism.

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A B

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F0/(F0-F)

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4

2 5

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2.0x10

4.0x10

5

6.0x10

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8.0x10

1/[GNPs]

Figure 6. Modified Stern-Volmer plot for the interaction of luminol with GNPs: triangular gold nanoprism (A) and gold nanodecahedra (B). For the static quenching process, the fluorescence data for the interaction between gold nanoparticles and luminol can be further analysed using the modified Stern-Volmer equation: 1 1 1 = + $ − "# # "# where fa is the fraction of accessible fluorescence and Ka is the the effective quenching constant which is analogous to the associative binding constant for the quencher-acceptor system

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.A

plot of F0/(F0-F) versus 1/[Q], shown in Figure 6, gives a straight line where the value of Ka can be found from the ratio of intercept and slope. The value of Ka has been found out to be 1.15×105


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L.mol-1 for triangular gold nanoprism and 1.66×105 L.mol-1 for gold nanodecahedra. This indicates that the associative binding constant of gold nanodecahedra is greater than that of triangular gold nanoprism which suggests that the gold nanodecahedra have more affinity towards luminol than that of triangular gold nanoprism. The interaction of gold nanoseeds with luminol (Figure S4, supplementary information), which are sphere in shape, gives an associative binding constant value of 8.91×104 L.mol-1 which suggests that both triangular gold nanoprism and gold nanodecahedra are having a greater interaction with luminol than that of spherical gold nanoparticles. The free energy change (∆G) of binding GNPs with luminol can be estimated using the Van’t Hoff equation: ∆% = −&'() * where K is analogous to the effective quenching constant Ka at the same temperature and R is the gas constant (8.314 J K−1 mol−1)

21b

. The value of ∆G was found to be −28.87 kJ mol-1 for

triangular gold nanoprism and −29.78 kJ mol-1 for gold nanodecahedra. This negative value of ∆G confirms the process of binding interaction between GNPs and luminol is spontaneous. 3.4. Energy transfer Förster non-radiative energy transfer or Fluorescence resonance energy transfer (FRET) is a non-destructive spectroscopic approach which can be used as the spectroscopic ruler to measure the distance over several nanometers. This FRET study was based on the assumption that all of the quenching arose from Förster’s energy transfer theory 36. According to the FRET theory, the energy transfer will take place under the following aspects: (i) the donor can produce fluorescence, (ii) the proximity and relative angular orientation of the transition dipoles of the donor and acceptor, (iii) fluorescence emission spectrum of the donor and UV–vis absorbance


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spectrum of the acceptor have more overlap (Figure S5, supplementary information) and (iv) the distance between the donor and the acceptor (+ )

21a, 31a, 37

. The efficiency of energy transfer

between the donor and acceptor, E, could be calculated by the following equation: , =1−

&= . & + +-

where & is the critical distance when the efficiency of energy transfer is 50% and can be calculated using the equation &- = 8.8 × 1023 4 5 26 78 9 : where k2 is the spatial orientation factor related to the geometry of the donor and acceptor dipoles (2/3 for fluid solution), N is the averaged refracted index of the medium in the wavelength range where spectral overlap is significant (DMF, 1.441 at 25°C) 38, 78 is the fluorescence quantum yield of the donor (luminol in DMF, 0.26)

39

and J is the overlap integral of the fluorescence

emission spectrum of the donor luminol and the absorption spectrum of the acceptor GNPs (Figure S4, supplementary information). The value of J can be calculated using the equation ∑

Gold Triangular Nanoprisms and Nanodecahedra: Synthesis and

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