稀有金属(英文版) 2019,38(12),1178-1186

Design and fabrication of a new fluorescence enhancement system of silver nanoparticles-decorated aligned silver nanowires

Jian-Chao Wang Hong-Sheng Luo Ming-Hai Zhang Xi-Hong Zu Jie Zhang Yu-Xin Gu Guo-Bin Yi

School of Chemical Engineering and Light Industry,Guangdong University of Technology

Guangdong Provincial Key Laboratory of Advanced Coatings Research and Development,China National Electric Apparatus Research Institute Co.,Ltd.

作者简介：*Guo-Bin Yi e-mail:ygb702@163.com;

收稿日期：23 January 2018

基金：financially supported by the National Natural Science Foundation of China (No.51273048);Science and Technology Planning Project of Guangdong Province (No.2017B090915004);the Open Operation of Guangdong Provincial Key Laboratory of Advanced Coatings Research and Development (No.2017B030314105);

Design and fabrication of a new fluorescence enhancement system of silver nanoparticles-decorated aligned silver nanowires

Jian-Chao Wang Hong-Sheng Luo Ming-Hai Zhang Xi-Hong Zu Jie Zhang Yu-Xin Gu Guo-Bin Yi

School of Chemical Engineering and Light Industry,Guangdong University of Technology

Guangdong Provincial Key Laboratory of Advanced Coatings Research and Development,China National Electric Apparatus Research Institute Co.,Ltd.

Abstract：

A new substrate,aligned Ag nanowires decorated with silver nanoparticle composite structure(AgNWs@AgNPs),was fabricated to investigate metalenhanced fluorescence(MEF) and its mechanism.The new composite structure was fabricated via a three-phase interface assembly method followed by SnCl₂ sensitization and AgNO₃ reduction process.The size and distribution of the nanoparticles on silver nanowires increased with the sensitization and reduction cycles.The formation of AgNPs on the surfaces of AgNWs was confirmed by multiple characterization methods including scanning electron microscopy(SEM),transmission electron microscope(TEM),atomic force microscopy(AFM) and X-ray diffraction(XRD).The results show that the fluorescence intensity of the poly(3-hexylthiophene)(P3HT) on the composite structure was greatly enhanced compared with that on bare glass substrate,and the intensity increased with the increase in particle sizes and density.The mechanism was basedo n the increase in excitation rate and the radiation decay rate.The new type of substrate could serve as a good and efficient MEF substrate for high-performance fluorescence-based devices.

Keyword：

Fluorescence enhancement; Aligned Ag nanowires; Self-assembly; Mechanism;

Received： 23 January 2018

1 Introduction

Since the first experimental demonstraation by Drexhage and coworkers [ 1, 2] ,metal-enhanced fluorescence (MEF)has received much attention.Fluorescence-based devices have been widely used in the field of photonics,chemistry,biology,medicine and materials science [ 3] .The explanation for a significant enhancement can be based on three mechanisms:(1) a local electromagnetic field enhancement activates the adjacent fluorescent molecules,resulting in an increase in the excitation efficiency [ 4] ;(2) the radiative decay rate of the fluorophore is enhanced combined with an increase in the quantum yield and decreased lifetime of fluorescent species [ 5] ;and (3) when the emission wavelength of fluorophore is close to the surface plasmon resonance (SPR) wavelength of nanoscale metals,energy transfer between the fluorophore and the metal results in a radiation integration of excited-state fluorescent species and surface plasma,and thus,fluorescence intensity is enhanced [ 6] .

A set of methods,such as electron-beam lithography,nanosphere lithography,vapor deposition and electrochemical deposition,block copolymer self-assembly and self-assembly [ 7, 8, 9, 10, 11, 12] ,were used to fabricate ordered nanostrueture arrays as MEF substrates.However,most of these methods are complicated,of high cost or difficult for large-scale applications.Therefore,simnple,economical and large-area fabrication of efficient MEF substrates with uniform and dense periodic nanoarrays was still a challenge.Bae et al. [ 13] reported the enhanced fluorescence of high-aspect-ratio silver nanowires.Goh et al. [ 14] produced etched Ag nanowires to create more hot spots along the long axis direction of Ag nano wire for sensitive surfaceenhanced Raman scattering (SERS) detection.High curvature points created by the increase in surface roughness serve as new generated hot spots which broke the limitation of Ag nanowires with limited hot spots at the tips.Dai et al. [ 15] reported Ag nanowire and nanoparticle hybrid system.All these substrates exhibited a stronger SERS or fluorescence signal.Interfacial self-assembly method is a silnple and efficient method [ 16, 17] for the fabrication of various nanofilms of noble metal nanostructures.Tao et al. [ 18] used Langmuir-B lodgett technique to fabricate aligned silver nanowire films which served as excellent SERS substrates with large electromagnetic field enhancement factors.Chen et al. [ 16] fabricated aligned Ag nanowire films as uniform and repeatable SERS substrates by direct two-phase interfacial self-assembly method.Herein,self-assembly process was used to assemble aligned films of silver nanowires.The resulting planar plasmonic substrate was used as MEF substraates with high fluorescence enhancement.

Metal-enhanced fluorescence effect occurred at the surfaces of plasmonic nanostructure wwas mainly due to the concentration of electromagnetic near fields,which were associated with strong localized surfaace plasmon resonances [ 19] .The resonant oscillation of the surface plasmons could cause a large increase in the electromaagnetic field.Most recently,the development of nanoparticles on one-dimensional (1D) nanomaterials attracts gregreat interest in various fields [ 20, 21, 22, 23] .Among them,silver nanowire decorated with silver nanoparticles has received great attention in surface plasmon resonance.Tran et al. [ 24] investigated optical properties of silver nanowires densely decorated with silver nanoparticles,and the hybrid structure exhibited well-separated and fairly regular spots of localized emission/scattering.Wang et al. [ 25] fabricated novel 1D Ag NWs-Ag NPs substrate for detection of protoporphyrin IX based on metal fluorescence enhancement.The intensity enhancement mechanism was simply attributed to the coupling effect of Ag nanowires and Ag nanoparticles.Research results from Michaels et al. [ 26] showed that strong electric fields were located at the junction of two Ag nanoparticles.A series of studies showed that when a metal nanoparticle was placed in the near proximity of another metal nanoparticle/film with a gap of a few nanometers,electric fields in the gap were greatly enhanced [ 27, 28, 29, 30, 31] .The mechanism may be derived from the coupling of surface plasmon polaritons of a metal film with localized surface plasmons of metal nanoparticles [ 32] .The large electric fields resulted in strong fluorescence and Raman enhancement of probe molecule in such a given site.

In this work,aligned silver nanowires decorated with silver nanoparticle (AgNWs@AgNPs) were fabricated using SnCl₂ sensitization and AgNO₃ reduction process without using any organic linking molecules.The mechanism of fluorescence enhancement of conjugated polymer poly(3-hexylthiophene)(P3HT) on silver nanoparticle-decorated aligned silver nanowires substrates was systematically investigated.The new type of substrate could serve as good and efficient MEF substrates for highperformance fluorescence-based devices.

2 Experimental

2.1 Materials

AgNO₃ (99.8%,Sinopharm Chemical Reagent Co.,Ltd.,China),poly(vinylpyrrolidone)(PVP,average molecular weight of 58,000),copper (Ⅱ) chloride dihydrate,ethylene glycol (EG),SnCl₂ (anhydrous) and hydrochloric acid were purchased from Shanghai Aladdin Bio-Chem Technology Co.,Ltd.P3HT (average molecular weight of M_w=30 kg·mol^-1) was purchased from J&K Scientific Co.Ltd.,China.All the reagents above were used as received without any purification.Milli-Q water and absolute ethanol were used in all preparations.

2.2 Materials preparation

2.2.1 Fabrication of aligned Ag nanowire substrate

The AgNWs were synthesized by the polyol method [ 33] .In the polyol process,silver nitrate was reduced at 160℃with ethylene glycol as the solvent and reducing agent and PVP as wire-directing agent.Self-assembled AgNWs were prepared via a three-phase interface assembly [ 21] ,First of all,aqueous suspension of AgNWs (5 ml) was added to the liquid surface of 25 ml chloroform in a glass vessel.An interface was formed between two immiscible liquids.One milliliter acetone was added slowly.Minutes later,a sparkling mirror-like surface emerged.The ordered Ag nanowire films were then transferred onto glass slide.Residual solvent on the aligned Ag nanowires substrates was removed by natural volatilization at room temperature.

2.2.2 Decoration of aligned Ag nanowires with silver nanoparticle

The obtained self-assembled AgNWs films were immersed in a mixed aqueous solution of SnCl₂ (0.02 mol·L^-1) and HC1 (0.02 mol·L^-1) for 2 min and were then rinsed in nanopure water.Then,the above SnCl₂-sensitized aligned AgNWs films were immersed in 0.02 mol·L^-1 aqueous solution of AgNO₃ for 5 min and were then rinsed in nanopure water.The above preparation process was repeated four times.The samples were labeled as AgNWs,AgNWs@AgNPs-1,AgNWs@AgNPs-2,AgNWs@AgNPs-3 and AgNWs@AgNPs-4 representing the reaction cycles of 0,1,2,3 and 4,respectively.

2.2.3 Preparation of P3HT films

P3HT films on quartz glass and obtained AgNWs@AgNPs substrates were prepared by spin coating.In the typical fabrication process,30μl P3HT chlorobenzene solution(5 mg·ml^-1) was spin-coated at a speed of 2000 r,min^-1for 60 s.All samples were dried in a vacuum oven for at least 12 h to remove the residual solvent.The samples were labeled as P3HT,S0,S1,S2,S3 and S4 and then kept in dark to prevent deterioration until measurement.

2.3 Characterizations

Extinction spectra were obtained on a Shimadzu ultraviolet(UV)-2450 (Japan) spectrophotometer.The investigations were performed by atomic force microscope (AFM,JEOL JSM-7600F,Bruker),transmission electron microscope(TEM,JEM-1400PLUS,Hitachi) and X-ray diffractometer(XRD,X'Pert Powder,Holland) with Cu Kαradiation.The fluorescence spectra were measured on a FluoroMax-4Spectrofluorometer (HORIBA Jobin Yvon,France).The samples were excited by a Xe lamp at 557 nm for steadystate measurements.The fluorescence emission was measured between 490 and 800 nm.Raman spectroscopic analysis was performed on a laser Raman spectroscopy(HORIBA Jobin Yvon) equipped with an Ar ion laser as the excitation resource (633 nm).

3 Results and discussion

3.1 Fabrication of silver nanoparticle-decorated aligned Ag nanowires

The process for the preparation of AgNWs@AgNPs is schematically illustrated in Fig.1.The fabrication process consists of four steps:containing alignment in the interface,transferring onto the substrates,SnCl₂ sensitization and AgNO₃ reduction.Sn²⁺could be adsorbed on the surface of the AgNWs in which AgNWs were coated by a thin layer of PVP,which contains amine and carbonyl groups,serving as“molecular anchors”that bond the Sn²⁺to the surfaces of the membrane [ 34] .In addition,there are plenty of hydroxyl groups on the surface silver nanowires synthesized by polyol process [ 35] .When exposed to mixed solution of SnCl₂ and HCl,the surface hydroxyl groups may be deprotonated,and negative oxygen sites were formed.Thus,Sn²⁺would be bound to negatively charged surfaces via electrostatic attraction.In the AgNO₃ reduction,the spontaneous oxidation of surface Sn²⁺to Sn⁴⁺and reduction of Ag+to Ag⁰ took place,as shown in E_q.(1):

Fig.1 Schematic illustration of AgNWs@AgNPs substrate fabrica-tion process

SEM images of aligned AgNWs and AgNWs@AgNPs are shown in Fig.2.It is shown that the surface of AgNWs is smooth,and AgNWs are parallel to each other,forming a close contact,and highly arrayed structure.Some large intervals might be caused during the transfer of films from interface to substrate.For AgNWs@AgNPs,small silver seeds were formed along the longitude surface of AgNWs without destruction of the arrangement of Ag nano wires.With the further increase in reaction cycles,silver seed grew,and new silver seed was generated.Silver nanoparticles were uniformly immobilized along the long axis direction.Almost every AgNW was covered densely and uniformly with a great quantity of nanoparticles,leading to the shrinkage of gaps.It was observed that the size and distribution of silver nanoparticles on AgNWs substrates were controlled by the cycle times.

The angles between silver nanowires and horizontal line were measured through the ImageJ.In Fig.2b-f,77%-85%nanowires were aligned in one direction±5°(Table 1).Standard deviation (σ) provides the degree of dispersion (Table 1).

where N is the number of silver nano wires,X_i is the angle between nanowire and the horizontal direction andμis the arithmetic mean of the angle of all the silver nano wires.The results appeared to reflect that most of the silver nanowires were parallel to each other and only a few silver nanowires were in disorder which are formed during the transfer of films from interface to substrate.

3.2 Morphology and crystallinity

To further analyze the size and morphology of silver nanoparticles decorated on silver nanowires and the interface areas of two kinds of nanoparticles,TEM experiments were performed.Figure 3 shows typical TEM images of as-synthesized Ag nanowires and AgNWs@AgNPs.The surface of as-synthesized silver nano wires was very smooth.After one sensitization and reduction reaction cycle,many silver nanoparticles appeared on the surface of Ag nanowires along the long axis.The average diameter of AgNPs was about 36.8 nm.The size and number of AgNPs grew with sensitization and reduction cycles,and the diameters were 45.6,53.3 and65.8 nm,for AgNWs@AgNPs-2,AgNWs@AgNPs-3 and AgNWs@AgNPs-4,respectively.Besides,the formed Ag nanoparticles were more uniform and denser on the surface of the nano wire with more reaction cycles.Moreover,it was obvious that there were large interface areas between AgNPs and AgNWs,suggesting that Ag nanoparticles were firmly anchored to the surface of Ag nanowires even after the ultrasonic process during sample preparation for TEM,and only a few unbond AgNPs were observed.An HRTEM image of AgNW s@AgNPs-1 shows the simultaneous presence of different crystallographic planes (Fig.3f),showing the expected spacings of 0.254 and 0.204 nm for(111) and (200) crystal planes of Ag.

Fig.2 SEM images of a,b aligned AgNWs film and c-f AgNWs@AgNPs-1,AgNWs@AgNPs-2,AgNWs@AgNPs-3 and AgNWs@AgNPs-4,respectively

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Table 1 Frequency of (μ±5°) and standard deviation of nanowires in Fig.2b-f

Fig.3 TEM images of a AgNWs and b-e AgNWs@AgNPs-1,AgNWs@AgNPs-2,AgNWs@AgNPs-3 and AgNWs@AgNPs-4,respectively;f HRTEM image of AgNWs@AgNPs-l

Figure 4 shows AFM height images of as-synthesized Ag nanowires and AgNPs-decorated Ag nano wires.Morphological differences between two types of nano wires were clearly observed.The as-synthesized silver nano wire shows a very smooth surface along the long axis,whereas for the AgNWs@AgNPs,the formation of AgNPs on the surface of aligned Ag nanowires can be clearly distinguished.A large amount of silver nanoparticles coated with Ag nanowires show a highly roughened surface along the longitudinal axis.AFM height images of aligned substrates are presented in Fig.4c,d to provide direct topographical comparisons between both types of substrates.Almost all the Ag nanowires were uniformly decorated with AgNPs,forming a new type of highly roughened nanowire without adverse effect on alignment of Ag nanowires.

Figure 5 shows typical XRD patterns of AgNWs and AgNWs@AgNPs-4.Both XRD patterns have five distinct diffraction peaks,and the peak positions of both types of AgNWs were consistent with each other.The diffraction pattern for the AgNWs@AgNPs-4 film has five peaks at38.10°,44.38°,64.43°,77.84°and 81.33°,corresponding to(111),(200),(220),(311) and (222) of fee silver according to JCPDS No.04-783.The diffraction peaks were sharp and intense,and no other peaks can be observed.It was indicated that AgNWs and newly formed AgNPs were not oxidized during and after the reaction and preserved high purity and good crystalline nature.

Fig.4 AFM topography of single as-synthesized a AgNWs and b AgNWs@AgNPs-1 on silicon substrate.AFM images of c aligned AgNWs substrates and d aligned AgNWs@AgNPs-3 substrates

Fig.5 XRD patterns of as-synthesized Ag nano wires and AgNWs@AgNPs-4

3.3 Metal-enhanced fluorescence

Figure 6 shows fluorescence emission spectra of P3HT on bare glass substrate and different AgNWs@AgNPs substrates.For comparison,the intensity of spectrum of P3HT was magnified to five times of its original data.Two emission peaks of P3HT were observed at 660 and 725 nm,respectively.The fluorescence intensity of the P3HT on bare glass substrate was the weakest,the fluorescence intensity on the aligned Ag nanowires was stronger,and the fluorescence emission on AgNWs@AgNPs was further increased.The fluorescence intensity of P3HT on aligned Ag nano wires was about 17.5 times higher than that on bare glass substrate.Compared with aligned Ag nanowires,enhancement factors of silver nanoparticle-decorated Ag nano wires substrates at 725 nm were 1.51,2.33,3.18 and3.88,respectively.It was indicated that the fabricated AgNWs@AgNPs substrate could significantly enhance the fluorescence emission for P3HT.Moreover,the intensity of P3HT was increased with the increase in size and distribution of nanoparticles.

Fig.6 Steady-state fluorescence spectra of P3HT films on bare quartz glass substrate and different AgNWs@AgNPs substrates excited by 557-nm excitation light source

3.4 Extinction spectra and excite enhancement

Figure 7 shows the evolution of ultraviolet-visible (UV-Vis) spectra of aligned AgNWs and silver nanoparticlesdecorated aligned silver nanowire substrates with increasing cycles of sensitization and reduction.The plasmon resonance peaks of silver nanowire located at about 322 and350 nm correspond to the transverse plasmon resonance and quadrupole resonance excitation of nanowires,respectively [ 36] .The SPR is extremely sensitive to size,morphology and arrangement of AgNWs [ 37, 38] .After one cycle of silver nanoparticle deposition,plasma resonance peaks of Ag nano wires changed obviously.The intensity of transverse plasmon resonance peaks declined clearly,and a new absorption bands appeared at about 450 nm.With increasing cycles of sensitization and reduction process,new peak strengthened continuously,indic ating the formation of silver nanoparticles.Furthermore,a slight red shift was observed,suggesting the size growth of the nanoparticles.It can be seen that the color of the substrates gradually changed from silver gray to dark yellow.Moreover,the formation of AgNPs on the surface of AgNWs also resulted in a tiny band shift of AgNWs to longer wavelength.It was demonstrated that there was the coupling of the AgNPs and AgNWs.The enhancement of the extinction intensity coupled with the emergence of new SPR of AgNPs contributed to larger overlap between SPR of AgNWs@AgNPs films and absorption peaks of P3HT (520,557 and 603 nm).Larger overlap was favorable for the enhancement of the excitation rate,which is beneficial to enhanced fluorescence of P3HT.

Fig.7 UV-Vis spectra of aligned as-synthesized nanowires and silver nanoparticle-decorated aligned silver nanowires with different sensitization and reduction cycles (1,2,3 and 4) and absorption spectrum of P3HT film on quartz

3.5 Energy transfer analysis

Metal-enhanced fluorescence is also strongly influenced by the energy transfer between the fluorophore and the metal substrate.When a fluorophore is excited,intramolecular energy transfer occurs from the excited fluorophore to the metal,resulting in metal nanoparticles to emit fluorescence with the same wavelength,and thus,fluorescence intensity is enhanced.The energy transfer from fluorophore to adjacent nanoparticle occurs only in the range of the SPR frequency [ 39] .Figure 8 shows that the emission peak positions of the P3HT film were outside the surface plasmon resonance region of the aligned AgNWs@AgNPs films,leading to a low energy transfer from the excited conjugated polymer P3HT to aligned AgNWs@AgNPs films.Therefore,energy transfer from fluorescence polymer to AgNWs@AgNPs substrate is quite negligible.

3.6 Electromagnetic field enhancement and Raman analysis

To further demonstrate the mechanism of MEF,surfaceenhanced resonance Raman scattering was also carried out in our work.Raman enhancement was mainly caused by local electromagnetic field enhancement [ 40] .Figure 9a shows the Raman spectra of the P3HT films on bare glass substrate and different AgNWs@AgNPs substrates.The characteristic Raman peaks at 715,1380 and 1450 cm^-1were assigned to the P3HT C-S-C ring deformation,C-C skeletal stretching deformation and C=C stretching.respectively [ 41] .It can be seen that the peak positions were consistent with different substrates,indicating no morphology or chemistry change in P3HT at the presence of AgNWs@AgNPs substrates.The spectroscopic band intensity was greatly enhanced by the inclusion of AgNWs@AgNPs,and the peak intensities were gradually enhanced with the increase in size and density of decorated nanoparticles.The observed Raman enhancement reflects the coherence enhancements in electromagnetic field due to the AgNWs@AgNPs,which may also contribute to the enhanced fluorescence.

Fig.8 Extinction spectra of aligned AgNWs@AgNPs-3 films and emission spectra P3HT film on quartz glass

Fig.9 a Raman spectra of P3HT films on bare glass substrate and different aligned AgNWs@AgNPs films;b illustration of electromagnetic field enhancement mechanism of MEF based on AgNWs@AgNPs substrate

Electromagnetic field of plasmonic nanostructure can alter the excitation and emission of fluorophore,and energy transfer between fluorophore and adjacent metal to enhance the fluorescence intensity [ 42] .The mechanism of the electromagnetic field enhancement is shown in Fig.9b.Tran et al. [ 24] studied the optical properties of a hybrid structure of a silver nanowire densely decorated with silver nanoparticles.Upon light radiation,regularly distributed spots of localized emission/scattering were observed along the AgNWs.The phenomenon was attributed to collective interactions of the localized surface plasmon resonances(LSPRs) of AgNPs with propagating surface plasmons(SPPs) on the AgNWs.The structure of our silver nano wire decorated with silver nanoparticles was similar to the system of Tran [ 24] .We make an analogy to show that our AgNWs@AgNPs substrates exhibit similar character.On the other hand,the enhancement of electromagnetic field in nanoparticle-nanoparticle or nanoparticle-nanowire gaps was much greater.Our AgNWs@AgNPs substrates have more hot spots than that from an inpidual nanowire through finite method [ 43] .Similarly,our silver nanoparticle-nanoparticle,nanoparticle-nanowire or nanowirenanowire gaps also contributed to total electromagnetic fields.So,for our silver nanoparticles-decorated aligned silver nanowire,electromagnetic field at these gaps was greatly enhanced.As a result,fluorescence and Raman intensity of P3HT were enhanced for electromagnetic field enhancement.

3.7 Fluorescence lifetime measurement

Enhancement of the radiative decay rate is related to metalenhanced fluorescence,usually accompanied by a decrease in fluorescence lifetime [ 44] .To further study the mechanism of the fluorescence enhancement of AgNWs@AgNPs substrates,time-resolved fluorescence spectroscopy was carried out.Figure 10 shows time-resolved decay curves of the P3HT films on bare glass substrate and different AgNWs@AgNPs substrates at 475-nm excitation.Monoexponential function was used to fit the experimental data.

Fig.10 Time-resolved decay curves of P3HT films on bare glass substrate and different aligned AgNWs@AgNPs films

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Table 2 Fitting lifetime of P3HT

Goodness of f_it was determined by R² value

whereτis the lifetime,A represents the value of weighting factor of fluorescence decay,t is the time and y₀ stands for weighting factor of nonradiative decay.Time-resolved decay curves of P3HT on bare glass are significantly different from the other curves,while for different silver nanowires substrates,there is no significant difference.The average lifetime of P3HT films on glass was found to be the longest,while for different AgNWs@AgNPs substrates,the fluorescence lifetime slightly shortened.Moreover,the fluorescence lifetime of P3HT is found to decrease with the increase in the number of fabrication cycles,and the data are listed in Table 2.This would partially demonstrate the observed fluorescence enhancement.The result shows that the emission enhancement is not only a result of increase in the absorption rate of P3HT,but also the result of the increase in radiative decay.

4 Conclusion

In this study,silver nanoparticles-decorated aligned silver nanowires (AgNWs@AgNPs) films were prepared as MEF substrates.Experimental results show that the size and distribution of nanoparticle were tuned by sensitization and reduction cycles.The fluorescence intensity of the P3HT on the fabricated substrates increased with the ihcrease in particle sizes and density.The mechanism of metal-enhanced fluorescent enhancement was based on the enhancement of the excitation rate from the larger overlap between SPR of AgNWs@AgNPs films and absorption peaks of P3HT,and the decrease in fluorescence lifetime of P3HT with the increase in the number of fabrication cycles.In a word,the fluorescence enhancement may be the result of an increase in both excitation rate and the radiation decay rate.Owning to the enhancement of fluorescence intensity and easy preparation,the silver nanoparticles-decorated aligned silver nanowire substrates are the suitable choice for good and efficient MEF sensing applications.

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