Haotian Ma,Liangjun Pan,Ji Wang,Li Zhang,Zhiling Zhang*

Key Laboratory of Analytical Chemistry for Biology and Medicine(Ministry of Education),College of Chemistry and Molecular Sciences,Wuhan University,Wuhan 430072,China

Key words:AgIn S2 Quantum dot Droplet microreactor Temperature Fluorescence property

ABSTRACT For the synthesis of AgIn S2 quantum dots(QDs),a suitable temperature is extremely important for control of the size,shape and fluorescence properties of QDs.Most of synthesis methods for AgIn S2 QDs are based on batch reactors,which bring uneven distribution of temperature,affecting their fluorescence propertiesand size uniformity.Here we designed a droplet microreactor with a temperature-controllable region,and successfully synthesized w ater-soluble AgIn S2 QDs.By accurately controlling temperature,we also studied how the reaction temperature affected the fluorescence properties of AgIn S2 QDs.The results showed that with the increasing of reaction temperature,the QDs size increased and the fluorescence peak constantly red-shifted along with enhanced fluorescence intensity.Based on the droplet microreactor,we could achieve more appropriate reaction condition to synthesize AgIn S2 QDs with higher fluorescence quantum yield(QY)and intensity.

Ternary I–III–VI2Quantum dots(QDs)have gained more and more attention due to their excellent biocompatibility,optical properties,and large absorption coefficient[1–5].Among them,AgIn S2QDs are a kind of direct-band-gap semiconductor and have adjustable optical band gap in the visible-to-near-infrared region.AgIn S2QDs with different structure,tunable spectral regions and small particle radius(less than 5.5 nm)can be synthesized by changing the ratio of precursors,reaction time or coating shell[6].So far,many methods have been reported to synthesize AgInS2QDs,such as hot injection[7,8],heating-up[9,10],solvothermal[11],hydrothermal and microw ave irradiation approach[12,13].During the synthesis of colloidal QDs,both size and shape of the QDs are affected by temperature because that the formation of nanocrystals requires adequate amount of thermal energy[6].However,most of the methods above are based on batch reactors,leading to uneven distribution of temperature,bringing poor homogeneity to the nanomaterials and low reproducibility between batches.Therefore,it is important to accurately control the temperature and realize the homogeneous nanomaterials synthesis.For AgIn S2QDs,it is necessary to study the temperature effect on fluorescence properties so as to provide a theoretical basis for high-quality QDs synthesis.

In recent years,ow ing to their fast mass/heat transfer,effective mixture of reagents,good reproducibility and better control of reaction conditions,micro fluidic reactors are show ing unique advantages over batch reactors,and have been more and more used in the synthesis of nanomaterials and related mechanism exploration[14–17].The development of microreactors is from the initial single-phase continuous fl ow to multiphase segmented fl ow.Continuous fl ow systems have been successfully applied to synthesize various materials,including metal nanomaterials(Au,Ag,Cu,among others),semiconductors(II–VI and III–V compounds)[18].However,in continuous fl ow s,the linear velocity of the liquid distributes unevenly in the same cross section,making it difficult to control the reaction time.Droplet microreactors make a homogeneous velocity,and reactants in droplets mix quickly,avoiding direct contact with channel w all[19].Shu et al.synthesized different-sized w ater-soluble Ag2S QDs that were NIR-emitting and visible-emitting at different temperatures in droplet microreactors[20].In addition,w hen integrated with specific functional units,researcherscan realize in situ monitor and accurate reaction control in droplet microreactors[20–23].Yashina et al.designed a tw o-stage droplet micro fluidic platform integrated with a real-time optical detection system for the synthesis of oilsoluble Cu In S2/Zn SNCs[15].

Fig.1.(a)Schematic diagram of the droplet micro fluidic chip.(b)Schematic diagram of w ater-soluble AgIn S2 QDs synthesis in a droplet.(c,d)Optical images of droplets in the fl ow-focusing and reaction region at 50?C.All the scale bars are 200 m m.

In this work,we designed a temperature-controllable droplet microreactor with more accurate and convenient temperature control than that in batch reactors and coupled with an optical fiber optic spectrometer to monitor the fluorescence spectra in situ.The micro fluidic chip was fabricated using soft lithography method.First,the structure with a fl ow-focusing region and a temperature-controllable reaction region was fabricated on a silicon w afer with 50 m m thick SU8 2050 photoresist.After that,the PDMS mixture of a 10:1 precursor/curing agent was poured onto the silicon master with about thickness of 5 mm.After curing at 75?C for 4 h,PDMS was peeled off from the master,and chip inlets and outlets holes were punched with a metal pipe.The temperature-controllable region was realized by a conductive area on ITO glass.A piece of ITO glass with designed structure was fabricated by soft lithography method and covered with AZ1500 photoresist.Then,9 mol/L HCl solution was used to etch the ITO glass,leaving the photoresist covered area unetched,and then the photoresist was washed off with ethanol solution.Finally,the PDMSwas bonded to the ITO glass with designed structure by O2plasma treatment to form a close chip,and the chip was baked at 120?Cfor more than 20 h to make the microchannels hydrophobic.A homemade temperature-controlling device was used to accurately control the local heating of the chip.Copper w ires were attached onto the ITO glass substrate with high purity silver paint.A PT-100 micro-thermistor(FMC 2101)located under the microfluidic chip was used as temperature sensor.

In the upstream region of the chip,tw o immiscible fluids were pumped into the microchannel by syringe pumps(pump 11 Pico Plus,Harvard Apparatus,USA).One of them was a fl uorinated oil continuous phase,the other was a disperse phase containing 5 mmol/L Na2S precursor solution and a mixture of Ag/In precursors solution (2.5 mmol/L AgNO3and 17.5 mmol/L In(NO3)3fully mixed with 150 m L MPA,and 0.25 mol/L NaOH was used to adjust the solution p H to about 7.0).The w ater-in-oil(W/O)droplets were formed at the fl ow-focusing structure.The size of the dropletscould be adjusted by changing the velocity ratio of the continuous phase to the disperse phase.The fl ow behavior of droplets was observed under an inverted microscope(TE2000-U,Nikon Corp,Japan)equipped with a CCD camera(Retiga 2000R,Qimaging Corp,Canada).

Fig.2.(a)Of fl ine fluorescence and absorption spectra of AgIn S2 QDs synthesized in droplets at 50?C,U W=20 m L/h,U O=40 m L/h.(b)TEM image of QDs.Inset:a highresolution TEM image.(c–e)XRD,EDX,FT-IR spectra.(f)The picture of a series of AgIn S2 fluorescent QDs synthesized at different reaction temperature(from left to right:30–70?C).

In the dow nstream region of the chip,once the droplets came into the heating area,the reaction inside the droplets began.And w hen droplets fl ow ing through the local reaction region,reaction conditions were changed by setting the temperatures of the reaction zone from 30?Cto 70?C.A 5-hour continuous synthesis was operated to investigate the stability of the chip,and fi ve chips were used under the same experimental conditions to investigate the reproducibility.A portable fiber optic spectrometer(QE65000,Ocean Optics,USA)coupled with the microscope was employed to monitor the fluorescence spectra of the products in situ.

The schematic diagram of the droplet micro fluidic chip was show n in Fig.1a.Tw o dispersed phases(one was a mixed solution of Ag/In precursors,the other was a solution of Na2S precursor)presented a stable laminar fl ow before arriving at the fl owfocusing structure.After that,W/O droplets were formed by the shear of the continuous phase(fl uorinated oil).The droplet micro fluidic chip allowed for the generation of monodisperse droplets of different sizes by adjusting the velocity ratio of the continuous phase to the disperse phase(Fig.S1 in Supporting information).Moreover,the droplets fl owed through the reaction region without interfusing with each other at 80?C(Fig.S1)and even a higher temperature(data not show n),indicating that droplet microreactors were stable enough for the synthesis of AgIn S2QDs under high-temperature conditions.

As the droplets fl owed through the reaction region,the synthesis of QDs within the droplets was continuously carried out.In this process,MPA acted as surface ligands to stabilize and control the QDs grow th.Online fluorescence spectra of AgIn S2QDs from individual droplets was obtained in situ with a portable fiber optic spectrometer.Fig.2a show s the fluorescence and absorption spectra of AgIn S2QDs synthesized in droplets at 50?C.QDs had a fluorescence peak at 615 nm and full w idth at half-maximum(FWHM)of 110 nm demonstrating typical characteristicsof ternary QDs[24].In addition,QDs synthesized in droplet microreactors showed a slightly narrower FWHM and a more uniform size distribution than that in the fl ask(Fig.S2 in Supporting information),show ing better ability in size control.Fig.S3(Supporting information)showed fluorescence spectra of QDs at different positions of the microchannel.The peak intensity increased with the residence time and tended to be stable,and the peak position remained unchanged.The residence time of the droplets in the w hole chip was less than 1 min,so the reaction time was much shorter than that in the fl ask(1 h).The size of the QDs was about(3.9?0.6)nm,with the narrow size distribution and good shape-uniformity con fi rmed by TEM image(Fig.2b).The high-resolution TEM image showed obvious crystal lattice fringes.Fig.2c showed the XRDpatterns of the QDs,the strong broad peaks around 2u values of 24.2?,26.6?,28.3?,44.5?and 48.1?attributed to(120)/(200),(002),(121)/(201),(040)/(320)and(123)phases of orthorhombic AgIn S2(JCPDSNo.25-1328).The broadening diffraction peaks suggested the small sizes of QDs.EDX measurement in Fig.2d con fi rmed the presence of Ag,In and S elements with an atomic ratio of 1:1.2:1.9,which was close to stoichiometry of AgIn S2.From the FT-IRspectra(Fig.2e),the bands 1570 cm?1and 1396 cm?1could be assigned to the C-O symmetric and asymm etric vibration of COO?group and the bands 2923 cm?1and 2852 cm?1referred to the asymmetric and symmetric stretching vibrations of C-H of MPA.The band around 1273 cm?1attributed to the bending vibrations of CH2-S and the absence of a peak at 2490 cm?1suggested the inexistence of free SH-,which demonstrated that MPA molecules were bound to the surface of AgIn S2QDs as ligands.The XPS result also illustrated that the valence states of the ions were Ag+,In3+and S2?,certifying the formation of AgIn S2in the droplet microreactors(Fig.S4 in Supporting information).The AgIn S2QDs achieved QYs of0.8%–8.8%with Rhodamine 101 as a reference standard(Table S1 in Supporting information).The maximum QY could reach 8.8%w hen the reaction temperature was 70?C.

Fig.4.(a)Time evolution of online fluorescence spectra of AgIn S2 QDs synthesized in droplets.Reaction conditions were fi xed at 50?C,U W=20 m L/h,U O=40 m L/h.(b)Online fluorescence spectra of AgInS2 QDs synthesized in different batch chips keeping reaction conditions fi xed at 52?C,U W=20 m L/h,U O=40 m L/h.

Temperature often plays a key role during colloidal QDs synthesis.In this work,the effect of temperature on AgIn S2QDs fluorescence properties was studied by changing the reaction temperature from 30?C to 70?C,while all other experimental conditions remained the same.3IF]igs.3a and b show the UV–vis absorption spectra and normalized fluorescence spectra of the products at different temperatures.There were no obvious absorption peaks,which might be ascribed to the w ide size distribution or trap-state-related emission [25].Meanwhile,reaction temperature also influenced the fluorescence intensity of QDs.When the reaction wasoperated below 30?C,no signi fi cant fluorescence emission was found(data not show n),indicating that the synthesis of AgIn S2QDs could not be realized at such low temperature.When the reaction temperature increased from 30?C to 70?C,the fluorescence peak of the QDs constantly red-shifted from 590 nm to 720 nm and the QY was also enhanced(Table S1),therefore,the fluorescence intensity was improved(Fig.3c).TEM images in Figs.3d–f also showed that the particle size increased from 3.5 nm to 4.4 nm as the temperature increased(30,50,70?C).However,w hen synthesized at a higher temperature(80?C),QY decreased to 5.8%,the fluorescence intensity of the QDs decreased obviously.We speculated that w hen reaction temperature was low,the system could not provide enough energy for ordered atom rearrangement on the surface of the QDs,leading to more surface defects and low fluorescence QY and intensity.As the reaction temperature increased,the surface defects decreased and the fluorescence intensity increased.But w hen the reaction temperature was too high(80?C),the grow th rate of nanocrystals became too fast to control,resulting in fluorescence QY and intensity decreased[6,26].Therefore,appropriate reaction temperature could lead to higher QYand higher fluorescence intensity of AgIn S2QDs.

Furthermore,we continuously recorded the fluorescence spectra of QDs products within the droplets at a fi xed position of the chip for 5 h.Fig.4a show s that there was no obvious change,indicating good stability of the droplet microreactors.Besides,fi ve chips were used for synthesis under the same conditions,and the fluorescence spectra of the products was nearly the same,show ing good reproducibility of the droplet microreactors(Fig.4b).

In conclusion,the temperature-controllable droplet microreactor with good stability and reproducibility was designed to synthesize the w ater-soluble AgIn S2QDs,and the temperature influence on the fluorescence properties of AgIn S2QDs was studied.Results indicated that the monodispersed orthorhombic AgIn S2QDs with MPA as surface ligands were successfully synthesized in droplet microreactors,and the reaction time was shorter than that in the fl ask.When reaction temperature increased from 30?C to 70?C,QDs size ranged from 3.5 nm to 4.4 nm,and the fluorescence peak constantly red-shifted from 590 nm to 720 nm along with enhanced fluorescence QY and intensity.AgIn S2QDs with the maximum fluorescence intensity and the QY of 8.8%could be obtained at 70?C.

In addition,droplet microreactors showed unique advantages in the synthesis of AgInS2QDs which were simple and able to precisely change and control the reaction conditions.Moreover,for the ternary QDs,droplet microreactors might better regulate the reactant composition and reaction conditions,thus effectively balancing the reactivity between the tw o cations,and synthesizing ternary QDs with less defects and higher fluorescence ef fi ciency.

Acknow ledgm ents

This work was supported by the National Natural Science Foundation of China(Nos.21375100,21775111)and the National Science and Technology Major Project of China (No.2018ZX10301405).

Appendix A.Supplem entary data

Supplementary data associated with thisarticle can be found,in the online version,at https://doi.org/10.1016/j.cclet.2018.04.033