- KHOSRO ZANGENEH KAMALI
In this work, a silver@graphene oxide (Ag@GO) nanocomposite-based optical sensor was developed for the detection of biomolecules such as dopamine (DA), ascorbic acid (AA), and uric acid (UA). An aqueous solution of Ag@GO was prepared using a simple chemical reduction method, and it showed a characteristic surface plasmon resonance (SPR) band at 402 nm. The SPR features of the Ag@GO nanocomposite were used for the detection of DA, AA, and UA. The SPR intensity-based limits of detection (LoDs) of DA, AA, and UA were 49 nM, 634 nM, and 927 nM, respectively. The SPR band position-based LoDs of DA, AA, and UA were 30 nM, 1.64 ïM, and 2.15 ïM, respectively. The present optical sensor was more sensitive to DA than to UA and AA. The interactions of the biomolecules with Ag@GO were studied based on the density functional theory (DFT), and it was found that DA had more interaction than AA and UA. This novel Ag@GO nanocomposite is simple to prepare and showed excellent stability and sensitivity toward the detection of biomolecules.
The similar material is used for colorimetric detection of Mercury(II) ions (Hg(II)) that is able to show existence of 100 µM Hg(II) ions in solution by naked eyes. The development of this optical sensor for Hg(II) using silver nanoparticles (Ag NPs) is based on the decrement in the localized surface plasmon resonance (LSPR) absorption of the Ag NPs and the formation of silver-mercury (AgHg) amalgam. It is observed that increasing Hg(II) ions concentration in the solution results in the decrease of LSPR intensity and decolouration of the solution. The existence of GO prevents the agglomeration of Ag NPs and enhances the stability of the nanocomposite material, enabling this material to be used in industrial and real sample applications.
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Di sini, oksida perak @ graphene (Ag @ GO) berdasarkan nanokomposit-sensor optik telah dibangunkan untuk mengesan biomolekul seperti dopamine (DA), asid askorbik (AA), dan asid urik (UA). Larutan akueus Ag @ GO telah disediakan dengan menggunakan kaedah pengurangan kimia yang mudah, dan ia menunjukkan satu ciri plasmon permukaan resonans (SPR) band di 402 nm. Ciri-ciri SPR daripada Ag @ GO nanokomposit telah digunakan untuk mengesan DA, AA, dan UA. Had keamatan-pengesanan (LoDs) bagi SPR berdasarkan daripada DA, AA, dan UA adalah masing-masing 49 nM, 634 nM, dan 927 nM,. The band SPR berdasarkan kedudukan-LoDS daripada DA, AA, dan UA adalah masing- masing 30 nM, 1.64 uM, dan 2.15 uM. Sensor optik masa kini adalah lebih sensitif kepada DA daripada UA dan AA. Interaksi daripada biomolekul dengan Ag @ GO dikaji berdasarkan ketumpatan teori fungsional (DFT), dan didapati bahawa DA mempunyai interaksi lebih daripada AA dan UA. Novel ini Ag @ GO nanokomposit adalah mudah untuk menyediakan dan menunjukkan kestabilan yang sangat baik dan kepekaan terhadap pengesanan biomolekul.Bahan yang sama telah digunakan untuk pengesanan ”colorimetric” ion Mercury(II), (Hg(II)) yang mampu dilihat dengan kewujudan 100 Î¼M ion Hg(II) dalam larutan dengan mata kasar. Pembangunan sensor optik bagi Hg(II) menggunakan nanozarah perak (Ag NPS) adalah berdasarkan pengurangan pada penyerapan Ag NPs resonan plasmon permukaan setempat (LSPR) dan pembentukan amalgam perak-merkuri (AgHg). Dapat diperhatikan bahawa peningkatan kepekatan ion Hg(II) memberikan hasil pengurangan pada intensiti LSPR dan perubahan warna. Peningkatan jumlah ion Hg(II) pada satu tahap membawa perubahan dalam morfologi Ag NPs dan pembentukan amalgam AgHg yang mempengaruhi LSPR Ag NPS dan menjadikan perubahan warna pada Ag@GO. Kehadiran GO menghalang penggumpalan Ag NPS dan meningkatkan kestabilan bahan nanokomposit yang membolehkan bahan ini untuk digunakan dalam industri dan aplikasi sampel sebenar.
Table of Contents
Figure 2: (A) Absorption spectra obtained for Ag@GO nanocomposite upon each addition of 100 nM DA. (B) Plot of absorption intensity vs. DA concentration. (C) Plot of Id vs. DA concentration. (D) Plot of λmax vs. DA concentration.
Figure 3: (A) Absorption spectra obtained for Ag@GO nanocomposite upon each addition of 5 µM AA. (B) Plot of absorption intensity vs. AA concentration. (C) Plot of Id vs. AA concentration. (D) Plot of λmax vs. AA concentration.
Figure 4: (A) Absorption spectra obtained for Ag@GO nanocomposite upon each addition of 5 µM UA. (B) Plot of absorption intensity vs. UA concentration. (C) Plot of Id vs. UA concentration. (D) Plot of λmax vs. UA concentration.
Figure 9: Absorption spectral changes observed for the Ag@GO nanocomposite (A) before and (B) after the addition of 200 µM Hg(II) ions. Inset: The digital photographic images taken for the corresponding solution.
Figure 10: (A) Absorption spectral changes observed for Ag@GO nanocomposite upon each addition of 100 nm μM of Hg(II) ions to the solution. (B) Plot of changes in the absorption intensity maximum at λLSPR of Ag@GO nanocomposite against various Hg(II) ions concentr
Figure 11: (A) Schematic explain the function of GO in the detection Hg(II) ions. (a) Addition of Hg(II) ions into a solution containing Ag@GO nanocomposite. (b) Adsorption of Hg(II) ions on the surface of GO. (c) Interaction of Hg(II) ions with Ag NPs and formation of AgHg amalgam. (B) Schematic representation for the formation of AgHg amalgam and its influence in absorption spectra of the Ag NPs present in the Ag@GO nanoparticles.
Figure 15: Cyclic voltammograms recorded in 0.1 M phosphate buffer solution with pH 7.0 at a scan rate of 50 mV s−1 for the GC electrode coated with the solution containing Ag@GO nanocomposite (A) before and (B) after addition of 200 µM Hg(II) ions.
Figure 16: Difference in percentage of Ag NPs absorbance peak reduction observed for Ag@GO nanocomposite in the presence of 200 µM Hg(II), Na(I), K(I), Mn(II), Ni(II), Zn(II), Co(II), Cu(II), Fe(II) and Fe(III) into the individual solutions. Inset: Photograph taken after the addition of 200 µM of Hg(II) ), Na(I), K(I), Mn(II), Ni(II), Zn(II), Co(II), Cu(II), Fe(II) and Fe(III) into the individual solution.
LoD limit of Detection
LSPRlocalized surface plasmon resonance
SPRsurface plasmon resonance
SERSsurface enhanced resonance plasmon scattering
HPLChigh-performance liquid chromatography
Hg(II) ionmercury (II) ion
rGOreduced graphene oxide
GCEglassy carbon electrode
DFTdensity functional theory
HRTEMhigh resolution transmission electron microscope
XPSX-ray photoelectron spectroscopy
FESEMfield emission scanning electron microscope
For several decades, silver (Ag) nanoparticles have been attracting attention because of their excellent optical and electronic properties, high catalytic activity, and biocompatibility. Hence, they are used in a wide range of applications such as catalysis 1, solar cells 2,3, and optical 4 and electrochemical sensors 5. Ag nanoparticles possess a sharp absorption in the visible region (400–500 nm), depending on the size of the nanoparticles. This absorption feature arises from the surface plasmon resonance (SPR), which is the absorption of light by the nanoparticles because of surface vibrations between atoms 6–8. This SPR feature allows Ag nanoparticles to be used in optical sensors for the detection of toxic metals 9, biomolecules 10, and organic compounds 11. The addition of any analyte to the Ag nanoparticles leads to assembled/aggregated nanoparticles. This influences the SPR absorption band and is extensively used to follow the molecular recognition processes.
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Dopamine (DA) is an important catecholamine that belongs to the family of excitatory chemical neurotransmitters. It plays an essential role in the functioning of the drug addiction, cardiovascular, renal, central nervous, and hormonal systems, and in Parkinson’s disease 12. Thus, it is very important to develop a simple sensor for the detection of a sub-micro-molar concentration of DA. In recent years, the detection of biomolecules such as uric acid (UA) and ascorbic acid (AA) in human fluids such as urine and serum has gained considerable attention 13. A deficiency or excess amount of UA in the body causes several diseases, including Lesch/Nyhan syndrome, hyperuricaemia, and gout 14. Cardiovascular disease and kidney damage result from an elevated UA concentration in serum 15. Analytical methods such as high-performance liquid chromatography (HPLC) 16, spectrofluorimetry 17, spectrophotometry 18, mass spectrometry 19, and electrochemical sensors 20 have been reported for the detection of these neurotransmitter molecules. However, the existing detection methods have several limitations such as the need for expensive equipment, well-trained operators, and tedious sampling and time-consuming procedures. Alternatively, an optical sensor platform is more attractive for sensing a wide range of analytes. It is cost effective, portable, has a rapid response, and can provide real-time analyses. Recently, Ramaraj and his coworker reported a silicate-Ag nanoparticle-based optical sensor for the detection of DA, UA, and AA with LODs of 5, 5, and 1 nM, respectively 9 (Figure 1).
Most commonly, Ag nanoparticles are synthesized using various chemical and physical methods, which are not eco-friendly and suffer from problems that include the poor reproducibility and stability of the Ag nanoparticles due to colloidal aggregation 21. In order to overcome such limitations, considerable efforts have been made to prepare Ag nanoparticles on polymer 22, silicate sol-gel 11, and graphene nanosheets 23. Among these, Ag-graphene has a large surface area and strong van der Waals force between the graphene and Ag nanoparticles, which significantly reduces nanoparticle aggregation. In addition, the high interfacial interactions ensure the stability of the Ag nanoparticles 24. In this study, graphene oxide-supported Ag nanoparticles were prepared using a simple chemical reduction method and used in an optical sensor for the detection of biomolecules such as DA, AA, and UA (Figure 2). The present synthetic method for the for preparation of Ag@GO nanocomposite has advantages over other methods 11, 21-24 such as, long term stability, high homogeneity, rapid and ease of preparation and avoids any surfactant, stabilizers.
Silver nanoparticles (Ag NPs) attracted much attention due to its biocompatibility, high catalytic activity, anti-bacterial activity, electronic and optical properties 25–28. The Ag NPs possess a principal absorption band in the region of 400 nm due to the localized surface plasmon resonance (LSPR)29,30. This SPR feature of Ag NPs is aroused due to the collective oscillation of electrons on the surface of the Ag NPs that are excited by incident electromagnetic waves 31. The SPR band position and intensity mainly depend on the size, shape and refractive index 32. This SPR band of Ag NPs is more sensitive to the surrounding environment and it significantly influences the band position and intensity. Based on the changes in the LSPR band position and intensity, an optical sensor platform with Ag NPs was developed to detect the wide range of analytes, including biomolecules 33, nitroaromatics 34, phenolic compounds 35, and heavy metal ions 36.
Among the investigated analytes, heavy metal ions especially Hg(II) ions are more often monitored with Ag NPs through the optical sensing method owing to its high toxicity and solubility in water37. Mercuric (Hg(II)) ions are mainly released into the atmosphere from solid waste incineration, power plants, and bumping fossil fuels38 that pollute water, soil and air 39,40. The existence of Hg(II) ions in water causes serious damage to the brain, nervous system, kidneys and endocrine system of living organisms41. Developing a system for detecting Hg(II) with high sensitivity and selectivity against other common metal ions dissolved in water is a challenge in recent years 42–47. From an environmental point of view the development of an inexpensive, simple, selective and sensitive method of detection of Hg(II) becomes highly important.
There are many types of sensors invented to detect Hg(II) in the environment. Some studies reported the detection of Hg(II) ions using electrochemical methods 48,49. Although they achieved a very high limit of detection (LoD), they need to use expensive apparatus and complicated setup. For the electrochemical testing of Hg(II) sample, the fabrication of an electrode is necessary, and that is a very tedious process. In addition, the detection of Hg(II) using fluorescence spectrometry has been widely investigated 50,51. Although this method is simple and is able to detect trace amounts of Hg(II) ions in solution, it requires expensive equipment to work. In this respect, colorimetric sensors are cheaper and do not require tedious preparation methods, colorimetric sensors have the advantage that the existence of Hg(II) is easily discernible to the naked eye without being affected by other possible dissolved ions 52–59. Recently, Hg(II) ion sensing was reported with noble metals such as Au and Ag by utilizing the size/interparticle distance-dependent optical properties and high extinction coefficients 60–62. The interaction between surfactants and metal NPs results in changes to the refractive index of these NPs and the LSPR band 32,33. They also may electrostatically repel the analyte, preventing it from interacting with the metal NPs and reduces the sensitivity of the sensor. In this study, the Ag NPs was prepared by using a simple chemical reduction agent and stabilized on graphene oxide (GO) sheets. Subsequently, the Ag@GO nanocomposite was used to develop a colorimetric sensor for the detection of Hg(II) with the naked eye and an optical sensor also developed based on the LSPR changes upon the addition of various Hg(II) concentrations. This significant change in the LSPR of the Ag NPs is due to change in the morphology through the formation of AgHg amalgam. Selectivity in the detection of Hg(II) in the presence of various environmentally relevant metal ions was also studied.
Most probably gold nanoparticles (NPs) synthesized in the 5th or 4th century BC in China and Egypt regions 63. From that time, gold NPs have been used in both medicine and aesthetic aspects. As the result of the interesting optical properties of gold NPs, they were used for changing color of glass 64, pottery and ceramics 65. Faraday got interested about the optical properties of gold NPs and reported about the range of colors of gold nanoparticles colloidal solutions from ruby red to amethyst in 1857. Then he studied the factors influencing the color of gold NPs solutions and concluded that ‘‘the mere variation in the size of particles gave rise to a variety of resultant colors’’. Other than their optical properties, many applications of metal nanoparticles have been found in biochemistry, catalysis and sensors. For instance, one of the anti-odour commercialized devices in Japan is using the technology of immobilizing gold nanoparticles in oxide matrixes as active oxidation catalysts 66. In defenitions, nanoparticles are particles composed of number of atoms, ranging from 3 to 107 67. Nanoparticles feature properties are different from atoms or bulk material due to their size. The metallic nanoparticles larger than 2 nm possess a strong and broad absorption band in the UV-visible spectrum that is called surface plasmon resonance (SPR) band. This absorbance has discovered by Gustav Mie and known as Mie resonance 67. For smaller nanoparticles, quantum effects become more prominent and LSPR disappears. All metal nanoparticles possess the mensioned optical property, but the series of Au, Ag and Cu have very intense LSPRs. Other than that, their easy synthesis methods and their robustness to environmental conditions made silver and gold NPs to widely be used in this field. The LSPR features such as position, shape and intensity are strongly depends on various factors, to mention: the changes in the interparticle distance of the NPs and , and the changes in the refractive index of the local surrounding environment 68.
There are indeed other types of plasmonic signals, such as the surface plasmon resonance band produced by planar metallic films, in reflection or transmission, some of them being called plasmon polaritons. Though the resulting physics is extremely exciting and the recent discoveries numerous, it is out of the scope of present thesis and they will not discuss further.
In recent years, many theories were adopted by both physicists and chemists in order to give a clear description of the SPR band and on the main factors impacting its position, broadness and intensity. Many works has done to overview the existing plasmon band theories 67, and explain the SPR band by Mie and effective medium theory 69. There are some researches to explain optical propertiese of NPs with arbitrary shape by Maxwell equation theory 6.
The phenomenon on absorbance of certain wavelength of light observed in transmission of light through metal nanoparticles in solid or solution phase, is called localized surface plasmon resonance band (LSPR).Nanoparticles intract with incident light in certain frequency that result global scattering of it. This observation can be explained by the collective resonance of the conduction electrons of the nanoparticle, due to interaction of electrons in nanoparticles with light. The evaluation of all parameters of material, specially its dielectric constant is necessary for understanding and study this phenomena. Usually, dielectric constant of nanoparticles count same as its bulk form and confinement effects and defects induced by edges or impurities will be neglected. For this aim a study on electrostatics in bulk metal by using Maxwell equations is necessary. In formulating the dielectric constant with known parameters, the Drude model, which describes the motion of free electrons in a metal can be applied. Then the question of the nanoparticles will be addressed: the conditions for conducting electrons resonance will be determined by several means. The determination of the frequency of the absorption maximum (denoted , the frequency of the Mie resonance), the height of this maximum and the width of the peak will be the ultimate goal of the calculations. The different geometrical confinement effect of free electrons on each material caused the electronic motion for nanoparticles vary material by material. Indeed, here the nanoparticles can be seen as a cationic network in which a cloud of conducting electrons (or free electrons) moves and oscillates. Nanoparticles dimensions are very small compared to the wavelength of the UV-visible light for which the phenomenon is observed and also comparable to the mean free path of electrons. The surface plasmon band is known to the resonance of the electronic cloud with the incident wave and the mechanics of this phenomenon can be evaluated.
In the case of nanoparticles, the conditions that electron cloud can resonate needs to be calculated. For this aim, The dielectric constant of metal nanoparticles assumed to be the same as the bulk material. Some postulates then become incorrect, but in calculation we have to keep them as an approximation. For instance, the electron density in small particles (r = 0) is not uniform and the charge will accumulate in particle edges and surface. Other than that, since the size of nanoparticles is very small comparing to the wavelength of incident light, we can consider that all electrons in the nanoparticle face with the same field at the given time and the electric field is independent of position 67. This hypothesis is known as the quasi-static approximation.
When the electric field incidents to the particle, it result the displacement of the electron cloud that leads to the creation of surface charges. The positive charge would be where the cloud is lacking and the negative charge would be where it is concentrated (Fig. 1). 67 The therm “surface” is justified by the electron cloud charge mentioned in previous statements. However, we have to kep in mind that all the electrons are moving together (collectively) under the influence of electromagnetic field. This collective oscillation leads to plasmon polaritons, 64 that is different with the free plasmon in the bulk metal.67
The term “plasmon” was given to the SPR phenomenon by Shopper, due to the bounded gaseous plasmon oscillations.67 The dipolar charge repartition imposes a new force on the electron cloud. The electrons undergo a restoring force which conflicts with the external electric field.
Figure.0.1. Schematic description of electronic cloud displacements in nanoparticles under the effect of a electromagnetic wave.
Recently, chemically modified graphene (CMG) has been studied in the context of many applications, energy-related materials, such as polymer composites, ‘paper’-like materials, field-effect transistors (FET), sensors, and biomedical applications, due to its excellent electrical, mechanical, and thermal properties. 70–72 Chemical modification of graphene oxide, which is generated from graphite oxide, has been a promising route to achieve mass production of CMG platelets. Graphene oxide contains a range of reactive oxygen functional groups, which renders it a good candidate for use in the aforementioned applications (among others) through chemical functionalizations.
Although graphene known a relative novel material of broad interest and potential,1,3 GO has a history that extends back many decades to some of the earliest studies involving the chemistry of graphite.4–6 The first, the British chemist B. C. Brodie was exploring the structure of graphite by investigating the reactivity of f
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