Silicon Quantum Dots Metal Enhanced Photoluminescence by Gold

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C: Energy Conversion and Storage; Energy and Charge Transport

Silicon Quantum Dots Metal Enhanced Photoluminescence by Gold Nanoparticles in Colloidal Ensembles: Effect of Surface Coating Juan José Romero, Jose H. Hodak, Hernán B. Rodríguez, and Monica C. Gonzalez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09310 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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The Journal of Physical Chemistry

Silicon Quantum Dots Metal Enhanced Photoluminescence by Gold Nanoparticles in Colloidal Ensembles: Effect of Surface Coating Juan José Romeroaƒ, José H. Hodakb, Hernán B. Rodrígueza, and Mónica C. Gonzaleza* a

Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), CCT-La

Plata-CONICET, Universidad Nacional de La Plata, La Plata, Argentina. b

Departamento de Química Inorgánica, Analítica y Química Física & INQUIMAE (UBA-

CONICET), Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón 2, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina ƒ

Present address: Instituto de Química Biológica (IQUIBICEN), CONICET –

Departamento de Química Biológica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina. *Corresponding author E-mail: [email protected]

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ABSTRACT In the present study we report the photophysical properties of colloidal ensembles of silicon quantum dots (SiD) and gold nanoparticles (AuNP), particularly focused on investigating metal-enhanced photoluminescence effects. AuNP with different sizes, (27 ± 10) and (88 ± 12) nm, and ca. 3.4 nm size SiD with different surface groups, either covered with an oxidized surface film bearing Si-OH surface groups or grafted with propylamine leading to Si-(CH2)2-CH2-NH2 terminal functionalities, were tested to evaluate gold enhancement of SiD photoluminescence. Nanoparticles were characterized by HRTEM, FTIR, XPS, ICPAES, and gel electrophoresis, while photophysical properties of nanoparticles alone and in colloidal ensembles at different concentrations were investigated by absorption and steadystate

and

time-resolved

photoluminescence

studies,

including

quantum

yield

determinations. Enhanced absorption and photoluminescence of SiD in the presence of AuNP was evidenced, leading in the most favorable cases to ca. 10-times increase in SiD brightness. This effect depends strongly on the SiD surface coating and its interaction with citrate-capped gold surfaces, where these interactions govern particles aggregation and relative distance distributions among SiD and AuNP in the ensembles. The nature of these interactions and how they affect metal-enhanced luminescence is thoroughly discussed. The present study provides significant information on the effect of SiD surface groups and surface charge on the metal-enhanced luminescence phenomenon in colloidal aqueous suspensions.


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INTRODUCTION Metal-enhanced fluorescence (MEF) is a class of phenomena that leads to a modulation of the fluorescence of a species via its interaction with intense local electric fields near a metal nanostructure. This property may be used to construct fluorescence sensors or to improve the emission yield of fluorescent devices.1,2 In particular, gold nanoparticles, AuNP, can promote MEF via their plasmon absorption band which generates a localized charge density oscillation known as surface plasmon resonance, SPR. The local electric field created around the metal surface by SPR is many orders of magnitude stronger than the optical excitation. The enhanced fields can dramatically increase the photoluminescence (PL) of nearby fluorophores by several mechanisms including the opening of additional excitation and emission pathways1–3 as well as the appearance of an additional strong net absorption of the fluorophores which also influences the fluorophore excitation rate.4,5 MEF depends critically on the spectral overlap between AuNP plasmon resonance and the emission and/or excitation spectra of the fluorophore as well as on the distance between them.4,6,7 In fact, the electrical field enhancement decays exponentially with the distance from the particle surface while a strong quenching of the dye emission takes place in the vicinity of the metal nanoparticle.8 As the separation between the emitter and the metal nanoparticle increases, the PL enhancement reaches a maximum at distances of ca. 10 nm, decreasing rapidly with further increase in separation distance.9 The actual distances between the chromophore and the metal nanoparticle at which MEF occurs is strongly determined by the chromophore orientation and the size, shape, number, and type of metal particles, as well as on the solvent. 3 ACS Paragon Plus Environment


Enhanced fluorescence by AuNP, though not through MEF mechanisms, was also observed for UV-emitting fluorophores, such as in chicken albumen thin films containing AuNP excited at 280 nm. This has been attributed to an energy transfer mechanism involving the simultaneous excitation of the fluorophore and gold d-band.10 MEF may occur on molecular fluorophores as well as on fluorescent quantum dots. Indeed, MEF by AuNP and Ag nanoparticles on semiconductor quantum dots involving different assembling strategies have been reported to strongly enhance dot emission.11–13 Si quantum dots (SiD) of 1-3 nm size have been proposed as suitable PL sensors in biological systems.14,15 Their high 400-600 nm PL upon excitation with 300-400 nm light, their biocompatibility, and their easy derivatization that lend them water solubility are their main attractive. In an attempt to improve SiD properties as PL sensors and photosensitizers in aqueous media, herein we investigate the MEF of ensembles composed of SiD and AuNP in aqueous solution. AuNP were selected for SiD MEF due to their high stability and biocompatibility.16 The main factors being considered are the AuNP size and SiD surface groups. Gold nanoparticles of 27 and 88 nm size, denoted as Au27 and Au88, respectively, were used along with SiD carrying two different types of surface groups: (1) an oxidized surface composed of Si-OH (here denoted as HOSiD) and (2) propylamine groups grafted to surface silicon atoms leading to Si-(CH2)2-CH2-NH2 terminal functionalities (denoted as PASiD). Because of the surface silanols groups, HOSiD is expected to show unspecific adsorption on AuNP surface, while the high adsorption affinity of NH2 groups towards gold17,18 provides PASiD with a stronger and more specific interaction with the AuNP surface.


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EXPERIMENTAL SECTION All the reactants and the standard equipment used (FTIR, IR-ATR, HRTEM, ICP–AES, XPS, and UV-Vis spectrophotometer) are described in the supporting information (S.I. Reactants and Equipment, respectively). It should be herein noted that XPS Au4f and N1s signals were fitted using Voigt peaks convolved with a Gaussian-Lorentzian of 1.7 eV width at half maximum (FWHM) representing the resolution of the XPS instrument and a background calculated by the Shirley approach. Synthesis and surface derivatization of Silicon Quantum Dots. SiD were prepared according to the methods published in the literature.19,20 Briefly, solid magnesium silicide (100 mg) was poured into a degassed 80 mM NH4Cl solution in anhydrous DMF and refluxed for 24 h under Ar atmosphere. The supernatant liquid contained 3.4 ± 0.6 nm in diameter SiD, which were purified by dialysis against DMF. The solvent was evaporated under reduced pressure and the remaining particles re-suspended in toluene. To obtain surface oxidized HOSiD, the “as obtained” particle suspensions were allowed to age under air at room temperature for three weeks. Propylamine derivatized SiD were obtained by adding allylamine (2% v/v) to a freshly-prepared deaerated suspension of SiD in toluene followed by 3 hours photolysis at room temperature with 254 nm light. The photolysis setup was constructed by placing eight Rayonet Lamps (RPR2537A, Southerm N.E. UltravioletCo.) around a cylindrical quartz reactor with magnetic stirring. The orangeyellow supernatant contains propylamine (PA) -terminated SiD (PASiD). In both cases, the solvent (toluene) was evaporated and the particles re-suspended in water and dialyzed against deionized water. SiD concentration was determined from measurements of the silicon content (mg/L) by ICP–AES and the average SiD molar mass estimated from the size distribution obtained 5 ACS Paragon Plus Environment


from HRTEM images and bulk silicon density (δSi= 2.33 g/cm3).15 As will be discussed further, SiD herein synthesized are amorphous and its density is expected to be ca. l.8 % lower than that of crystalline silicon.21 However, the latter difference is of no significance to the SiD molar mass thus estimated. Synthesis of Gold Nanoparticles. AuNP were prepared from the seed-growth method with hydroxylamine.22 AuNP seeds were obtained by a modification of the citratereduction method proposed by Turkevich– Frens.22,23 Briefly, 65 ml of 254 µM of HAuCl4 aqueous solution was heated to 100 oC and 6.5 ml of an aqueous 40 mM sodium citrate solution was added under continuous heating for 10 minutes. The obtained red suspension containing 27 ± 10 nm size spherical particles was dialyzed against deionized water and stored in the dark at 4 oC. Seed growth was performed by addition of 4.00 ml NH2OH (40 mM) to 7.5 ml of a 27 nm size particle suspension (3 nM) under continuous stirring at room temperature in a flask containing up to 75 ml ultrapure water. Addition of either 100 or 300 l of HAuCl4 (254 M) under constant stirring at room temperature lead to an immediate color change of the suspension due to the growth of AuNP.22,24 Suspensions were dialyzed against ultrapure water. AuNP concentration was determined from UV-Vis data following the method reported in the literature22 which considers the absorbance of pure AuNP aqueous suspensions at 450 nm and at the plasmon band maximum to estimate, both, AuNP concentration and size. Sizes obtained by this method differ in ca. 15% with those determined from HRTEM. AuNP are named Au27 and Au88 for particles of 27 ± 10 and 88 ± 12 nm size, respectively.


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In summary, the different types of particles investigated are: (i) citrate-coated gold nanoparticles (AuNP) of 27 and 88 nm size (Au27 and Au88, respectively); (ii) 3.4 nm size silicon quantum dots (SiD) coated with propylamine (PASiD) and surface oxidized SiD (HOSiD). A coarse representation of the particles surface is shown in Scheme 1. Scheme 1: Surface composition of PASiD, HOSiD, and 27 and 88 nm size AuNP. +

NH3 +

CH2

NH3

O O O

Si

O OH

Si

Si

CH2

O Si

Si

Si Si

O

+

Si Si

Si Si OH O Si O

O

Si Si

Si

Si

NH2

O

O

O Si OH O

O

CH2

Si

Si Si

NH3

O

Si

O

CH2

CH2

O

CH2

CH2

+

O

NH3

CH2

CH2

CH2 CH2 Si

CH2 Si Si Si Si

O

O

Si

Si

Si

CH2 CH CH2 2

Si CH2

CH2 Si

CH2

NH3

CH2 NH2

Si

Si

CH2O

CH2

Si

CH2

CH2 CH2

Si

CH2

Si CH2

OH CH2

CH2

CH2

CH2

O Si

Si

CH2

Si

Si

Si Si

O

HO

O

CH2 CH2

O +

NH3

O

O

O O

O

CH2

O NH2

O

O

O

+

NH3

HO O

CH2 CH2

O

O

O

O

Si

Si

Si

O

+

NH3 CH2 CH 2

CH2

Si

O

O HO

CH2 CH2

Si

+

HOSiD

Si Si Si

O

CH2

CH2

Si

+

NH3

CH2

+

NH3

CH2

Si

Si CH2

CH2

NH2 CH2

O

O

O

O

+

NH3

+

NH3

PASiD

Au27 or Au88

Preparation of AuNP and SiD mixtures. Known volumes (between 0 to 50 L) of a mother SiD suspension (8.5 M PASiD or 6.7 M HOSiD) where mixed at room temperature with 500 L of AuNP suspension (either 190 pM Au27 or 23 pM Au88). Mixture suspensions were of pH ca. 7.5 and 110-5 M ionic strength. To determine the importance of light scattering under the experimental conditions used in the ensembles, total transmittance spectra taken with an integrating sphere were compared with the attenuation spectrum obtained with a conventional spectrophotometer, as shown in S.I. Figure S1. Since the main differences observed are less than 10% at absorption wavelengths below 300 nm, it may be concluded that scattering if of negligible importance under the experimental conditions used. 7 ACS Paragon Plus Environment


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For the XPS and FTIR studies of SiD-AuNP ensembles, experiments were performed upon deposition and further evaporation of the suspension mixture on C and KBr supports, respectively. Physicochemical assays. Photoluminescence measurements were performed with a Jobin-Yvon Spex Fluorolog FL3-11 spectrometer. The fluorimeter is equipped with a 450 W Xe lamp as the excitation source, a monochromator with 1 nm bandpass gap for selecting the excitation and emission wavelengths, and a red sensitive R928 PM as detector. All spectra were corrected for the wavelength-dependent sensitivity of the detector and emission of the source. The emission spectra were corrected for Raman scattering by using the associated solvent emission spectrum. To estimate the relative photoluminescence quantum yields (M/o), the emission spectra of SiD-AuNP mixtures and that of pure SiD suspensions were collected exciting at 340 nm under identical experimental conditions (excitation, lamp energy, spectrometer band-pass, and temperature). The temperature was controlled to ± 0.1 oC with an F-3004 Peltier sample cooler controlled by a LFI-3751 temperature

controller

(Wavelength

electronics).

Time-resolved

luminescence

measurements were carried out in the same Jobin-Yvon Spex Fluorolog FL3-11 spectrometer with the aid of a time-correlated single photon counting (TCSPC) module. The excitation was done using a 341 nm LED (FWHM ~ 1.21 ns, respectively) operating at 1 MHz repetition. Data was globally fitted as a sum of exponentials including the instrument response function deconvolution, until optimal values of χ2, residuals, and standard deviation parameters were attained. Gel electrophoresis was performed using a 1% agarose gel in TAE buffer of pH 7.5 as support and a TAE buffer of pH 7.5 as moving phase. The electrophoresis was run for 30


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minutes at an applied constant voltage of 100 V. The nanoparticles were detected by their photoluminescence upon irradiation with UV light filtered with a SW06 filter from a GBox, Synoptics Ltd. Cambridge.

RESULTS AND DISCUSSION PASiD, HOSiD, Au27, and Au88 sole particles suspensions and the mixtures PASiDAu27, PASiD-Au88, HOSiD-Au27, and HOSiD-Au88 were characterized by several techniques and tested for MEF. Table 1 shows an outline of the different techniques used to study sole particles and particle ensembles. Table 1: Two-way table listing the techniques used to study sole nanoparticles and nanoparticle ensembles. See the Experimental section for the abbreviations of the techniques used. Sole nanoparticle Sole nanoparticle PASiD

   

ICP-AES XPS TEM FTIR

     

PL Time resolved PL UV-Vis absorbance ICP-AES HRTEM FTIR

 Gel Electrophoresis

HOSiD

 Gel Electrophoresis

 PL  Time resolved PL  UV-Vis absorbance

Au27

Au88

       

HRTEM XPS UV-Vis absorbance XPS TEM/EDS PL Time resolved PL UV-Vis absorbance

 HRTEM  UV-Vis absorbance

    

XPS TEM PL Time resolved PL UV-Vis absorbance





Characterization of sole SiD and AuNP Figure 1A shows the HRTEM micrograph of Au27, Au88, and HOSiD. Size distribution of Au27, Au88, and HOSiD follow a Gaussian statistics with mean diameters of 27 ± 10, 88 ± 12, and 3.4 ± 0.5 nm, respectively. Au27 images (Figure 1B) and electron diffraction Fourier transforms show the typical polycrystalline gold fcc structure. On the other hand, HOSiD HRTEM images (Figure 1C) shows an amorphous structure. Figure 1: (A) HRTEM and size distribution of Au27, Au88, and HOSiD; (B) HRTEM micrograph showing Au27 crystal planes of gold; (C) HRTEM image of amorphous HOSiD; (D) TEM images of Au27 seeded from pure 79 pM Au27 aqueous suspensions; (E) TEM images of Au27 seeded from aqueous suspensions containing 79 pM Au27 and 33M PASiD; (F) TEM images of Au27 seeded from aqueous suspensions containing 79 pM Au27 and 43 M HOSiD.

number of particles

number of particles

(A)

10

20 30 40 Feret diameter / nm

80

Au88

number of particles

number of particles

Au27

40 50 60 70 Feret diameter / nm

50 60 70 80 90 100 110 120 Feret diameter / nm

10 ACS Paragon Plus Environment 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Feret diameter / nm

10

number of particles

30

number of particles

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 30 40 Feret diameter / nm

HOSiD

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Feret diameter / nm

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(B)

(C)

HOSiD

Au27

2 nm

(D)

(F)

(E)

100 nm

100 nm

100 nm

The FTIR spectrum of HOSiD (see Figure 2A) presents signals in the 1040 - 1150 and 990-950 cm-1 regions characteristic of Si-O-Si and Si-OH vibrations, respectively, thus denoting surface-oxidized silicon quantum dots.25 On the other hand, PASiD spectrum shows bands in the 1640 and 1100 cm-1 regions characteristic of N-H bend and C-N vibrations of primary amines, respectively. The contribution of Si-O-Si vibrations to the peak at 1100 cm-1 cannot be discarded, as also supported by the presence of Si-OH characteristic peaks at 990 cm-1. The presence of bands at 1435 and 1260 cm-1 due to Si-C deformation in Si-CH2 and the peaks in the 2930 –2885 cm-1 region due to CH2 stretching and bending, confirm the attachment of propylamine groups to the surface of the silicon quantum dots, in agreement with previous reports.15,25 11 ACS Paragon Plus Environment


The XPS spectrum of PASiD (see Figure 3A) shows N 1s signals with binding nergy (BE) of 399.5 (70%) and 401.4 (30%) eV assigned to C-NH2 and C-NH3+X- environments, respectively.15 On the other hand, the Si 2p region displays the contribution of signals at 101.8 (71%), 103.0 (20%) and 104.0 eV (9%) which show Si-C, Si-O, and SiOx environments,26 indicating ca. a 30% oxidation of surface silicon. The area ratio of the N to Si peaks corrected by the atomic and instrument sensitivity factors yield an average N:Si surface ratio of 0.5:1, thus indicating an efficient coverage of the silicon quantum dots surface with propylamine groups. The presence of a low amount of Si-O moieties at the interface of PASiD assures that the photoluminescence properties of these quantum dots becomes independent on the silicon crystalline structure and organic coating,25,27 as will be described further in the text. Figure 2: (A) FTIR spectra of PASiD (top red spectrum, a) and HOSiD (black bottom spectrum, b). (B) Agarose gel electrophoresis of (from left to right) PASiD and HOSiD, Col 1 vs Col 2 Col 5 vs Col 6

respectively. (B) a

658

1395 1260 1100 990

1640

b 2940

(A)

Transmitance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

wavenumber / cm-1

Gel electrophoresis experiments using 1% agarose gels in TAE buffer of pH 7.5 clearly showed that PASiD and HOSiD move in opposite directions (see Figure 2B) 12 ACS Paragon Plus Environment

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consistent with a positive surface charge supported by NH3+ groups in PASiD and Si-O- in HOSiD. These observations further confirm an efficient functionalization of the SiD, as also supported by XPS experiments. Figure 3: (A) Si 2p and N 1s XPS region of PASiD; (B) AuNP4f and N 1s XPS signals of PASiD-Au27 ensembles; (C) XPS N 1s signal of PASiD-Au88 ensembles. (A) N1s

Si 2p

404 402 400 398 396

106

104

BE / eV

102

100

BE / eV

(B)

(C)

Au 4f

90

N 1s

N 1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


88

86

BE (eV)

84

82

404

402

400

398

396

BE (eV)

404

402

400

398

396

BE (eV)

The UV-Vis absorbance spectrum of HOSiD and PASiD are characteristic of 3 nm size silicon quantum dots, as shown in Figure 4. The spectrum of PASiD also shows a superposed band at 294 nm attributed to the propylamine coating. However, both silicon quantum dots show very similar emission spectra upon 340 nm excitation, as depicted in 13 ACS Paragon Plus Environment


Figure 4 inset, in agreement with the fact that the organic coating does not affect mild oxidized silicon quantum dot PL.25 The relatively broad emission spectrum of the silicon quantum dots strongly supports a broad distribution of particle sizes and possibly different surface morphologies.27 Plasmon resonance band peaking at 524.5 nm and 571.5 nm observed for citrate covered Au27 (158 pM) and Au88 (2.3 pM) in aqueous suspensions (see Figure 4) are in line with those expected for particle sizes of ca. 27 and 88 nm, respectively, as determined by HRTEM.22 Due to the high increase of the absorption coefficient of the SPR band with particle size,22 the concentrations of Au27 and Au88 used in the following experiments differ by ca. 63-68 times to allow for optical measurements and analysis. Figure 4: Normalized absorbance spectrum of 70 M HOSiD (full blue), 100 M PASiD (dashed red), 158 pM Au27 (dashed green), and 2.3 pM Au88 (full green) aqueous suspensions. The black arrow indicates the excitation wavelength used. Inset: HOSiD (full

Emission (a.u.)

blue) and PASiD (dashed red) emission spectrum obtained upon 340 nm excitation.

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

300

400

500

wavelength/nm

340

440

540

640

wavelength / nm


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Characterization of AuNP-SiD ensembles Low resolution TEM micrographs of Au27 seeded from pure 79 pM Au27 aqueous suspensions and the same suspension after the addition of 43 M HOSiD and 33M PASiD (see Figures 1 D, E, and F, respectively) were obtained to gain information on aggregate formation.28 In both cases, SiD concentration is in high excess respect to Au27. In fact, while Au27 in the absence of SiD are highly homogeneously dispersed, addition of HOSiD induces the predominant formation of small closed Au27 aggregates. More remarkable, addition of PASiD leads to the formation of Au27 aggregates with a clear opened 3D structure. In line with these observations, EDS spectra taken for solvent-evaporated PASiDAu27 mixtures, see S.I. Figure S2, show the presence of PASiD and Au27 in the same region of space. To further explore the nature of PASiD and AuNP interactions, the XPS spectrum of PASiD-Au27 ensembles where obtained. The band corresponding to Au4f shows the contribution of a main species satisfying the characteristic 3.7 eV separation of the Au4f level, with Au4f7/2 binding energy (BE) of 84.2 eV (93%) attributed to Auo, as shown in Figure 3B. A second less significant contribution of ca. 7% is observed at 85.5 eV. Considering that Cl 2p signals were also observed (see S.I. Figure S3), this contribution may be assigned to Au+ present as Aun[AuCl2]- surface groups.29,30 Corresponding N 1s signal analysis (see Figure 3B) denote the contribution of peaks at 399.9 (57%) and 401.6 eV (43%) assigned in the literature to C-NH2 environments15 and to more oxidized N environments like in C-NH3+X-,30,31 respectively. These same bands contribute to the XPS N 1s band of PASiD-Au88 ensembles (see Figure 3C) but with a proportion of oxidized nitrogen environment to neutral amine (399.4 (74%) and 400.8 eV (25%)) within the same 15 ACS Paragon Plus Environment


range observed for sole PASiD and on the order of published values for amine coated gold nanoparticles.32 However, a higher contribution of the oxidized N environment (43% against 25-30 %) is observed in the presence of Au27. The latter observation might indicate a specific interaction between PASiD and Au27 surface leading to a more oxidized N environment which is of little or no significance for Au88. PASiD-Au27 and PASiD-Au88 Ensembles Photophysical Studies Addition of different volumes of PASiD to an Au27 suspension (see Experimental) leads to the appearance of a new plasmon band around 640 nm which shifts to longer wavelengths at the expense of the initial 524.5 nm band of Au27, as shown in Figure 5A. At wavelengths below 350 nm, a linear dependence of the absorbance with [PASiD] is observed, as depicted for the absorbance at 340 nm (A340) in Figure 5A inset. However, the slope of the linear fitting of A340 vs [PASiD] is 2.5 times higher for PASiD-Au27 mixtures than for sole PASiD suspensions under otherwise similar conditions. Figure 5: (A) Absorption spectra of PASiD-Au27 mixtures. The black line shows the spectrum of 158 pM Au27 aqueous suspensions. The arrows indicate increasing [PASiD]:[Au27] ratios: 8.5×103, 25.4×103, 59.4×103, 84.8×103, and 169.6×103. Inset A: Absorbance at 340 nm as a function of [PASiD] for () PASiD-Au27 mixtures and () PASiD pure suspensions. (B) Absorption spectra of 2.5 pM Au88 (black line) aqueous suspensions in the presence of increasing amounts of PASiD. The arrows indicate increasing [PASiD]:[Au88] ratios: 5.5×105, 16.5×105, 38.4×105, and 54.9×105. Inset B: Absorbance at 340 nm as a function of [PASiD] for () PASiD-Au88 mixtures and () PASiD pure suspensions.


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1.5

1.5

1.0 0.5 0.0 0.0

10.0

20.0

[PASiD]/ 1.0

1.0

300

400

500

1.0 0.5 0.0 0.0

10.0

20.0

30.0

[PASiD]/ 0.5

Au88

Au27

0.0

Abs 340 nm

2.0

(B)

Absorbance

Abs 340 nm

(A)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600

700

800

wavelength / nm

0.0

300

400

500

600

700

800

wavelength / nm

On the other hand, addition of increasing amounts of PASiD to an Au88 suspension shows an augmented light absorbance at the shorter wavelengths increasing with PASiD concentration, but with no significant changes in the plasmon resonance band of Au88, as may be observed in Figure 5B. At 340 nm, the absorbance of PASiD-Au88 mixtures linearly increase with PASiD concentration (see Figure 5B inset). The slope of the linear fitting of A340 vs [PASiD] is about 2.5 times higher than that of sole PASiD suspensions, as also observed for PASiD-Au27 ensembles. The luminescence spectrum of PASiD aqueous suspensions obtained upon 340 nm excitation is independent on the presence of Au27 and Au88, as observed in S.I. Figure S4. However, the emission intensity increases with increasing [PASiD] in the mixture. To compare the effect of Au27 and Au88 on silicon quantum dots emission, relative emission quantum yields (M/0) were calculated according to equation (1), as proposed in the literature33 considering identical experimental conditions for the excitation and detection of the emission of silicon quantum dots in the absence and presence of gold nanoparticles. The integrals in the numerator and denominator represent the area under the emission spectra of 17 ACS Paragon Plus Environment


Page 18 of 36

the ensembles and silicon quantum dot sole suspensions, respectively, obtained upon 340 nm excitation. A340M and A340,0 represent the absorbance at 340 nm of the ensembles and pure silicon quantum dots suspension, respectively. The refraction index of the ensembles and of pure silicon quantum dots suspensions, n2M and n2o, are assumed to be identical because of the low concentration of metal nanoparticles used in the experiments.

Since Au27, Au88, and PASiD absorb light of 340 nm, the emission quantum yields of PASiD in Au27- and Au88- containing suspensions, M, were calculated using the absorbance at 340 nm assigned to PASiD in the sample (

) instead of A340M. To that

purpose, Au27 (or Au88) absorbance at 340 nm was subtracted from A340M assuming the absorbance of Au27 (or Au88) constant regardless of the increasing concentration of PASiD in the mixtures, as suggested by the linear dependence of A340M with [PASiD], shown in the insets of Figures 5A and 5B, respectively. M/o values thus calculated yield information on the metal enhancement or quenching effect on SiD emission. Obtained M/o values are < 1 for PASiD-Au27 ensembles but remain > 1 for PASiD-Au88, as shown in Table 2. However, due to the fact that both ensembles depict A340M > 0.1 at the excitation wavelength and in the wavelength range were emission occurs, the obtained M/o values are a low limit estimation of the real values due to inner filter effects (IFE) affecting both, PASiD absorbance and emission. The effect of IFE is that of reducing the emission intensity, as also observed for quenching processes, but originated in different causes. Therefore, it is necessary to compensate for IFE to obtain more realistic values of M/o.34 18 ACS Paragon Plus Environment

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Among the proposed corrections in the literature, that by Albinsson et al. (see eq.(2)) is the most widely accepted.33,35,36 (2) In this equation,

and

stand for the as obtained and corrected

fluorescence after removing the IFE ; Aexc and Aem stand for the absorbance at the excitation (λexc) and emission (λem) wavelengths, respectively. The introduction of eq. (2) in the intensity fluorescence integrals of eq.(1), yield a corrected relation of quantum efficiencies, (M/o)corr, as described in S.I. Corrections for Inner Filter Effects. Values of (M/o)corr for PASiD-Au27 and PASiD-Au88 ensembles are shown in Table 2. Interestingly, corrected values further stress the enhancement of PASiD luminescence by both, Au27 and Au88.

Table 2: M/o and [em] values of as obtained and corrected for IFE, see text, relative decay times (i/o) and as a function of [PASiD]:[Au27] and [PASiD]:[Au88]. [PASiD]:[Au27]

ΦM/Φ0

(ΦM/Φ0)corr

1/0

2/0

[em]corr

8,481 25,444 59,370 84,813 169,628

0.42 0.33 0.45 0.51 0.44

2.4 1.9 3.0 3.3 2.7

0.57 0.65 0.84 0.90 0.90

0.75 0.84 0.91 0.94 0.94

0.70 0.80 0.90 0.93 0.93

3.4 2.4 3.3 3.5 2.9

[PASiD]:[Au88]

ΦM/Φ0

(ΦM/Φ0)corr

1/0

2/0

[em]corr

548,819 1,646,456 3,841,732 5,488,188

1.6 1.5 1.3 1.3

3.5 3.6 3.4 3.5

0.60 0.64 0.51 1.05

0.82 0.87 1.05 0.83

0.77 0.81 0.72 0.93

4.5 4.5 4.7 3.8



Luminescence intensity decays (I(t)) were analyzed in terms of the multiexponential model33 taken as the sum of individual single-exponential decays i, as shown in eq.(3), where i stands for the amplitude

The photoluminescence decay of pure PASiD aqueous suspensions may be well fitted by three first order decays with lifetimes 1o = (3.5  0.1), 2o = (12.4  0.1), and 3o = (0.54  0.01) ns. While 1o and 2o may be assigned to the decay lifetimes of two different families of PASiD with emission maxima at 420 and 435 nm, respectively (see Figure 6), 3o is assigned to light scattering by the particles. Luminescence decay lifetimes of PASiDAu27 and PASiD-Au88 mixtures may also be well fitted by three components, the shorter lifetime assigned to light scattering. Lifetime components 1 and 2 strongly depend on the presence of Au27 and Au88 and are smaller than 1o and 2o (see Table 2). However, their associated spectrum does not change with the composition of the suspension, as shown in Figure 6 for PASiD-Au27 ensembles. Figure 6: Transient emission spectra associated to the short (1) and the long (2) lifetime (see text and Table 2 for corresponding values) obtained for different PASiD-Au27 concentration ratios: from bottom to top 8.5×103, 25.4×103, 59.4×103, 84.8×103, and 169.6×103. The dashed spectrum corresponds to a pure 1M PASiD suspension.


Page 20 of 36



2

500

500

435

1

420

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Emission Intensity (a.u.)

Page 21 of 36

wavelength (nm)

The quantum yield and the overall lifetime of a fluorophore can be expressed as a function of rate constants, as shown in eq. (4)33 where

and

stand for the

radiative and non-radiative decay rate constants, respectively. In the presence of gold nanoparticles and MEF,

is increased to

while

is consider not to

be modified,37 though this is certainly a coarse approximation.8 Moreover, for simplicity, it is assumed that emi  [em] is almost equal for the radiative processes associated to the short (1) and the long (2) lifetime components contributing to PASiD luminescence. Therefore, the average decay time observed for each [PASiD]:[Au27] and [PASiD]:[Au88] ratio calculated using eq. (5) and neglecting scattering is used, and [em] is calculated by eq. (6). The values obtained for [em] for the different ensembles are depicted in Table 2. Average [em] values of 3.1  0.5 and 4.4  0.4 obtained for [PASiD]:[Au27] and [PASiD]:[Au88] ensembles, respectively, are in the order of those reported for solution-based enhanced fluorescence sensing platforms.12,38

(5) 21 ACS Paragon Plus Environment


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(6)

HOSiD-Au27 and HOSiD-Au88 Ensembles Photophysical Studies Addition of increasing amounts of HOSiD to a suspension of Au27 (see Experimental section) shows an augmented dispersion on the mixture spectrum at the higher HOSiD concentrations and an increased absorbance at the shorter wavelengths (see Figure 7A). On the other hand, addition of HOSiD to an Au88 suspension mainly shows an augmented light absorbance at the shorter wavelengths and a much less significant absorbance diminution in the plasmon resonance band of Au88, as may be observed in Figure 7B. Also, the long term precipitation of Au27 and Au88 is visually observed. The absorbance at 340 nm (A340) of HOSiD-Au27 and HOSiD-Au88 mixtures increase linearly with [HOSiD], as shown in the insets of Figures 7A and 7B. The slopes of the linear fittings of A340 vs [HOSiD] for HOSiD-Au27 and HOSiD-Au88 ensembles are 1.7 and 1.6 times higher than that observed for sole HOSiD suspensions. Figure 7: (A) Absorption spectra of HOSiD-Au27 mixtures. The spectrum of a 57 pM Au27 (black full line) aqueous suspension is shown for comparison. The arrows indicate increasing [HOSiD]:[Au27] ratios: 28.3×103, 56.5×103, 113.1×103, and 141.4×103. Inset: Absorbance at 340 nm as a function of [HOSiD] for () HOSiD-Au27 mixtures and () corresponding HOSiD pure suspensions. (B) Absorption spectra of 0.6 pM Au88 (black lines) aqueous suspensions in the presence of increasing amounts of HOSiD. Inset: Absorbance at 340 nm as a function of [HOSiD] for () HOSiD-Au88 and () sole HOSiD suspensions. The arrows indicate increasing [HOSiD]:[Au88] ratios: 23.3×106, 93.2×106, and 116.5×106. 22 ACS Paragon Plus Environment

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(A)

(B)

0.4 0.0 0

10

0.5

20

30

40

[HOSiD]/M

1.0

0.6 0.3 0.0 0.0

10.0

20.0

30.0

40.0

[HOSiD] /M

0.5

Au27

0.0

A340

0.8

Absorbance

A340

1.0

0.9

1.5

1.2

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au88 0.0

300

400

500

600

300

700

400

500

600

700

wavelength / nm

wavelength / nm

The emission spectra of HOSiD aqueous suspensions in the presence of Au27 and Au88 upon 340 nm excitation are identical to those of pure HOSiD suspensions, as depicted in S.I. Figure S5. However, the emission intensity increases with increasing [HOSiD] in the mixture. As discussed previously, relative emission quantum yields M/o estimation from eq.(1) considers the absorption of HOSiD in the ensemble at 340 nm ( ). Thus calculated M/o values, shown in Table 3, show a wide variability within 0.8 and 2.1 with no systematic trends with the type of ensemble and [HOSiD]:[AuNP] ratio. Since steady state luminescence experiments for determining quantum yields were performed with diluted suspensions showing absorbance values < 0.05 in the wavelength range of interest, no IFE corrections were need in this case. The luminescence decay of pure HOSiD aqueous suspensions may be well fitted by four first order decays. The decay with lifetime in the order of fractions of ns is assigned to scattering while the other three may be assigned to different HOSiD chromophores, with lifetimes 1 = (1.4  0.4), 2 = (4.4  0.7), and 3 = (11.8  1) ns. Luminescence decay lifetimes of HOSiD-Au27 and HOSiD-Au88 mixtures may also be well fitted by four components, the shorter lifetime assigned to light scattering. Lifetime components 1, 2, 23 ACS Paragon Plus Environment


Page 24 of 36

and 3 are, within the experimental error, not sensitive to the [HOSiD]:[Au27] and [HOSiD]:[Au88] ratios, as also shown in Table 3. Values of [em] obtained using eq. (6) for the different ensembles are depicted in Table 3. Average [em] values of 1.4  0.5 obtained for [HOSiD]:[Au27] and [HOSiD]:[Au88] ensembles seem not to support a significant MEF of HOSiD by Au27 and Au88 colloidal suspensions. Table 3: M/o, relative decay times (i/o), , and [em] values as a function of [HOSiD]:[Au27] and [HOSiD]:[Au88].

[HOSiD]:[Au27]

ΦM/Φ0

1/0

2/0

3/0

[em]

28,271 56,543 113,085 141,356

2.1 1.3 1.3 0.8

1.10 0.93 0.90 0.93

1.10 1.04 0.98 0.96

0.98 0.98 0.93 0.88

1.04 0.98 0.99 0.96

2.0 1.3 1.3 0.83

[HOSiD]:[Au88]

ΦM/Φ0

1/0

2/0

3/0

[em]

23,309,984 46,619,968 93,239,936 116,549,920

1.2 1.7 1.6 1.0

1.03 0.91 0.91 0.89

1.05 1.01 1.03 0.99

0.95 0.96 0.99 0.96

1.02 0.98 0.99 0.98

1.2 1.7 1.6 1.0

Altogether, the results herein presented evidence some general trends depending primarily on the nature of the SiD surface and the AuNP size. Summarizing, HOSiD – AuNP mixtures show: (i) an absorbance diminution in the plasmon resonance band of Au27 and Au88 suggesting AuNP aggregation and long term precipitation by addition of HOSiD, in line with the observation of dense aggregates by TEM; (ii) an increased HOSiD absorbance at 340 nm by ca. 1.7 and 1.6 times in the presence of Au27 and Au88, 24 ACS Paragon Plus Environment

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respectively; (iii) average [em] values of 1.4  0.5 for, both, HOSiD-Au27 and HOSiDAu88 ensembles which strongly suggest minor metal-enhanced emission. The brightness of HOSiD in Au27 and Au88 aqueous colloidal systems is increased by ca. 2.4 and 2,2 times, respectively, mainly due to an increased absorbance. On the other hand, PASiD – AuNP mixtures show (i) the appearance of a new plasmon band suggesting a change in the Au27 aggregation induced by PASiD, in line with TEM analysis suggesting the formation of opened 3D aggregates; (ii) an augmented PASiD absorbance by ca. 2.5 times, both, in the presence of Au27 or Au88; (iii) average [em] values of 3.1  0.45 and 4.4  0.4 obtained for PASiD-Au27 and PASiD-Au88 ensembles in excess of PASiD, respectively. Therefore, the occurrence of MEF in PASiD-Au27 and PASiD-Au88 colloidal systems is strongly supported. As a consequence, PASiD brightness in Au27 and Au88 aqueous colloidal systems is increased by ca. 7.8 and 11 times, respectively, due to both, an absorbance and an emission enhancement. Considering that the PL of HOSiD and PASiD is regulated by the silicon core and not by their specific surface groups and that SiD emission spectrum overlaps with Au27 and Au88 plasmon bands, an enhancement of both SiD spontaneous emission rate is expected depending on the separation distance to AuNP. Therefore, the differences observed in the PL behavior of HOSiD and PASiD due to the presence of AuNP might depend on differences in the nature of their interactions. These interactions, regulated by the surface groups, lead to different AuNP aggregation morphologies, as supported by TEM micrographs and plasmon absorbance changes, vide supra, localizing the SiD at different mean distances of the metal nanoparticle.



Theoretically optimized geometries of aliphatic amine compounds on Au(111) surface indicate the presence of weak interactions between the nitrogen atom lone pair and a single gold atom of the substrate.39,40 Despite their weak nature, N-AuNP interactions were proposed to favor the self-assembling of gold nanoparticles on amino-functionalized surfaces.41,42 Moreover, reported DFT calculations suggest that binding sites occur most likely at the surface ridges and amine binding to AuNP surfaces is substantially stronger at the ridges than on the flat surface sections. Reported correlation studies on the radius of curvature of spherical gold nanoparticles of varying sizes with their respective thiol terminated oligonucleotide loading densities indicate that gold nanoparticles with diameters < 20 nm show a dramatic increase in surface coverage when compared to their larger counterparts, while AuNP with diameters > 60 nm mimic the properties of a planar surface. These suggestions are in line with our observations of a specific interaction between PASiD and citrate-capped Au27 surface leading to a more oxidized N environment which is of little or no significance for Au88. Displacement of surface citrate groups with multidentated PASiD might favor the interaction between different Au27 nanoparticles, which under the control of steric effects leads to the opened 3D structured agglomerates observed by TEM. Formation of such agglomerates was also reported in the literature for diimines and gold nanoparticles.43,44 A coarse representation of the thus formed aggregates is schematized in Scheme 2A. The latter observations are in agreement with the absorbance changes observed in the position of Au27 plasmon resonance band with increasing [PASiD], vide supra. The combination of surface plasmon resonances of nearby Au27 results in more concentrated electric fields at the interparticle gap.45 Such plasmon coupling causes a red-shifted scattering component of the aggregated particles, as observed in Figure 5A. 26 ACS Paragon Plus Environment

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On the other hand, Au88 resonance plasmon band is not modified by the presence of excess PASiD thus indicating that PASiD does not affect Au88 aggregation. However, the fact that low concentrations of Au88 are used in these experiments should not be disregarded. Considering the presence of NH3+ groups on PASiD surface and the negative charges on Au88, an electrostatic interaction between the particles surface groups is expected to maintain the particles in proximity,17,18 as supported by the enhanced absorption and emission of PASiD-Au88 ensembles. In fact, PASiD distribution around Au88 in aqueous suspension may be rudimentary envisaged as the primitive distribution model of positively charged multivalent colloid around a negatively charged surface46 for which an enhanced concentration with respect to bulk is expected, as depicted in Scheme 2C. It may be argued that specific interactions between the N amine groups of PASiD and Au27 bring the particles in very close proximity. Under such regime, near-field coupling between the emitter and the metal might take place leading to rapid photoexcitation energy dissipation by local heating of the metal. However, due to excess PASiD in the mixture, the simultaneous occurrence of electrostatic interactions between PASiD and Au27 (as observed for Au88) might be responsible for the observed enhanced emission. In fact, the lower [em] values observed for Au27 than for Au88 may be a consequence of PASiD specific interactions with Au27 though a dependence of MEF on the size of the gold nanoparticle cannot be discarded. Scheme 2: (A) PASiD specific interaction with Au27 surface modifies Au27 own distribution. (B) Effect of HOSiD on Au27 aggregation. (C) Schematization of PASiD (light blue circles) and HOSiD (yellow circles) particle distribution (dNi/Ni) with distance 27 ACS Paragon Plus Environment


Page 28 of 36

normal to the surface of AuNP. The distribution plot is a coarse representation following published studies on the distribution of polyvalent colloids. (A)

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O

CH2

CH2

O

Si CH2

Si CH2

CH2

O

O CH HO 2

CH2

Si Si

Si

Si Si

CH2O

CH2

O

NH2

Si Si

Si O

O

O

O HO

Si

Si Si Si

2

O

O

O

O

O

O

O

O

O

NH3 CH2 CH 2

CH2

SiSi

Si

Si

+

O HO O

O O O HO

O

O

Si

Si

Si OH O Si O

O

O

O

O

Si

O

O

O

O

Si

O Si OH

O OH

Si Si

Si Si Si

O O HO O

O O HO

Si

O O

O

O

O

Si

Si

CH2 CH2 OCH

NH3

O

O HO

O

O

Si

Si

O

Si

O O O O

O

O

O O

O O

+

Si Si

Si Si

O

O HO

O

O

OH

O O HO O

O

O

CH2

Si

O

Si

O

O

O

O

Si

CH2

CH2

Si

O Si

O O HO

O

O

O

O +

NH3

CH2 CH2

O

Si Si

CH2 Si Si CH2

CH2

CH2 CH2 O CH2

+

NH3

+

NH3

CH2

CH2

O Si

Si

Si O

O

O

O

O

O HO O

O

O HO

O Si Si

Si Si

Si

O O O

O O

O HO

Si

NH2

O

CH2

CH2 Si Si Si

+

NH3

CH2

O

CH2 CH2

O

O

+O

NH3

NH3 +

NH3

+

NH3

O

O

O HO

O

Si

Si

Si

HO

O

O

O

O

O

O

O

O

O O +

O

O

O

O

O

O O

O

O

O

O

O

O

O

+

NH3O

OH

O

O

NH2

O

NH2 +

NH3

O HO O

O

O

O

O

CH2

O

O HO

O

O HO

O

Si Si

O

O

O

O

O HO

O

OH

O

O

Si

Si Si OH O Si O

O O HO

O

O

O

Si

Si

Si Si

O

O HO O

O

O O

+

NH3

O

O

O

OCH 2 O

CH2 +

NH3

O

O HO

O

O O

O

Si OH O Si O

O Si

Si

O

O

O

OH O

O

O

O OH

Si Si

Si

O

O Si

Si

Si O

Si Si

O

Si Si

Si Si

Si

O HO

O OH

Si

Si

O

O O

O

O

+

NH3

CH2

O

CH2

CH2

O

O

CH2

Si CH2

Si CH2

CH2

CH2 NH2

Si

Si Si

CH2O

CH2

O

O

O O HO O

CH2

CH2

O

CH2

CHO2 H O +

NH3

CH2

CH2

CH2 CH2

CH2

CH2

Si CH2

Si

Si

CH2O

CH2

NH2

CH2

Si

Si

Si

Si

CH2

+

CH2 O

Si

Si

CH2 CH2

NH3

Si Si Si

CH2

+

NH3 CH2 CH 2

CH2 Si

NH2

OH

O O

O

O

O

CH2

Si Si

Si

Si

Si

O

CH2 CH2

O

Si

Si

Si

Si

CH2

Si SiSi

Si

Si

Si

O

OH O

O + NH3 CH2 CH 2

CH2 Si

Si

CH2

+

O +

NH3

CH2 CH2

CH2

CH2

Si CH2 O CH2 CH2 + O NH3O

Si

Si

CH2O

NH2

Si

Si

Si

CH2

O

Si

Si

Si

O

Si

Si

Si Si

O

O

O

CH2

2

O

O OH

Si

Si

O

O O HO

O

O

O

CH2 CH 2 CH2

O

O

O

O

CH2

Si SiSi

CH2 CH2

CH2

O O HO

O

O

NH3 CH2

CH2

Si Si Si

Si

Si

Si

+

NH3

CH2

Si Si

Si Si

CH2

CH2 CH CH2 2

+

NH3

CH2 CH2

O

Si

O CH2

CH2

Si

Si Si

Si

CH2 CH2

O

O

+

CH2 CH2

O

Si Si

O

CH2 Si

O

O

O

O

O

CH2 O O CH2 O CH SiO 2 H O CH

O O

O

O

O

O

Si OH O Si O

O

O

O

O

Si

Si Si

O

O HO

O

O

+

SiSi

O

O

O O

O O

O

O O HO

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HO

O

NH3

Si Si

Si

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O

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O

O

O

O

CH2

Si

Si

O

O HO +

NH3

NH2 CH2

CH2

Si Si

CH2 Si Si CH2

O

CH2

CH2

O

CH2 Si Si Si

CH2 Si Si CH2

CH2 CH CH2 2

+

NH3

O NH2

CH2

NH3

O O

CH2

CH2

O

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O

O

OH

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Si

O

O HO

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OH

O

O

O

CH2

CH2 Si

Si Si

Si

O Si OH CH2

NH3

+

CH2

CH2

CH2

CH2

O

CH2

CH2 Si Si Si

+

NH3 NH2

HO

O +

NH3

O

O

Si

Si

Si

+

CH2

CH2

NH3

O

O

O

O

Si

O

O

Si

Si Si

Si

Si O

O

Si Si

O HO

O

O O HO

O

+

NH3

CH2 CH2

O

Si

O

CH2 CH 2

NH2

+

CH2

CH2 CH2

O

OH O

CH2

Si CH2

CH O 2

+

O

NH3

+

NH3

O

O

O

O

CH2

NH3

CH2

CH2

CH2 O

O O HO

O

O

O

CH2

CH2

CH2 Si Si Si

CH2 Si

CH2

+

NH3

+

NH3

CH2

CH2 CH2

CH2

CH2

O

O HO O O

CH2

Si

Si CH2

Si CH2

CH2

CH2

O

O

O

O

O HO

CH2

CH2

CH2

CH2 NH2 CH2

CH2 O

NH2

CH2

CH2

O

NH3

+

CH2 Si

Si

Si Si

CH2O

CH2

+

NH3

O

O

O

Si Si Si

+

NH3

O

Si

Si

Si

Si

NH3

O

O O

NH3 CH2 CH 2

Si

Si

CH2

O O

Si

O

O Si

Si

Si OH O Si O

O

O

Si Si

Si Si

Si O

Si Si

O

O

O

O

O

+

O HO

+

Si Si

Si

Si O

O

O

O HO

SiSi

CH2 CH2

O

O

O

O

O HO

O

O HO

O

O Si

Si

O

O

O

CH2

Si

Si

Si

O

O HO O

O

O

+

Si

CH2 CH CH2 2

NH3 O

O

O HO

Si

Si Si

O O

O

O

O

CH2

CH2

O

O

O

O

O

O

NH3

CH2

Si

+

O O HO

O HO

O

O

+

CH2

O HO

O

O

O OH

Si

Si

O

O O

O

O

O

CH2

O

Si Si

CH2 Si Si CH2 O

CH2

CH2

O O

O

O

O

O

CH2

CH2

O

O

O O HO

O

O

NH3

NH2

CH2

NH3 O

O

O

HO

O

O O

O

O

O

O O

O

O

O HO

CH2

CH2

O

O

O

O

O HO

O

O

O

+

O

O

O

O O HO

O

+

+

NH3

O

O

O O

O

NH3

O

O HO

HO

O

O

NH3

CH2

CH2

O

O HO

NH2

CH2

CH2

CH2

O

O O

O

O O HO

O

+

NH3

CH2

CH2

Si CH2

Si CH2

CH2 NH2

O

CH2

Si

Si

Si Si

CH2O

CH2

+

NH3 O

O

O HO O

O

Si Si Si

CH2

CH2

O

O

O

O HO

CH2

Si

Si

Si

Si

O

O

O O HO

Si

CH2 CH2 CH2

O

CH2 Si

Si

O

O

O

O HO

+

NH3 CH2 CH 2

Si SiSi

Si

Si

Si

CH2 CH CH2 2

O O HO

Si

CH2

Si Si

Si

NH3

O O

CH2

CH2

Si

+

O O

O

+

NH3

CH2 CH2

O

Si Si

CH2 Si Si CH2 O

CH2

CH2

O

O

O

O

NH3

O

O

O

CH2

CH2 +

O

O O

O O

O

NH2

O

HO

CH2

CH2 Si Si Si

+

NH3

O

O

CH2

CH2

CH2

O HO

NH2

CH2

NH3

Si

Si

O +

NH3

O

O

OH

O

O

+

NH3

(C)

+

CH2

NH3

CH2 CH2

O

O O

O

O

O O HO

O

HO

CH2 NH2

3

CH2

CH2

+

NH3

O

O

CH2

Si

CH2 CH2

CH2

Si

+

CH2

SiSi

Si

Si Si Si

NH2

CH2

CH2

Si CH2 CH2

Si

CH2

CH2 CH2

CH2

CH2

CH2

CH2

O

Si

Si Si

CH2O

CH2

CH2

Si Si

Si

Si

Si

CH2

NH3 CH2 CH 2

Si

Si

Si

CH2 CH2 +

O

CH2

Si

Si

Si

NH3

Si Si

Si Si

O

NH3

O

+

CH2

O

Si Si

CH2 Si Si CH2

CH2

CH2 CH CH2 2

O

O

CH2

CH2 +

ONH

O O

O O

CH2

CH2 Si Si Si

+

NH3

O HO O

O

O HO O

NH2 CH2

CH2

O HO

+

NH3

NH2 +

CH2

NH2

O

CH2

CH2 +

NH3

O O

O

CH2

CH2

CH2

CH2 Si Si Si

Si

CH2

Si Si Si

CH2 NH O

NH3

CH2

CH2 +

O HO

O CH2

CH2

CH2

Si

+

+

CH2

O O HO

CH2

O

NH2 CH2

O

O

CH2

CH2

CH2 CH2 +

OH

NH3

O

Si Si Si

NH2

O CH2

Si

CH2

CH2

NH2

CH2

CH2

Si Si

CH2

NH3

CH2

NH3

CH2

CH2

O O O

Si

O OH

Si

Si

OH O

O Si

Si

Si Si

Si

Si Si O

O

O

x

O

O O O

O Si

Si

O

O Si

Si

O

Si

Si

Si Si

Si Si

Si O

Si

O

O

O O

Si

Si

O

OH

O

Si

Si

O Si

Si

O Si Si

Si

Si Si

Si

Si Si O

O Si

Si Si

Si

Si Si O

O

O

OH

O

O

O O Si

Si O

O Si

Si

Si O

Si

Si

O Si OH O Si O

Si OH O Si O

O Si

O Si Si

O

O OH

Si Si

Si Si

O Si

Si

Si

O OH

Si Si

Si

O

Si

Si

O

O Si Si Si

O

O

O

O

Si

O

O

OH

O

O

O OH

Si

O

O Si

O

Si

Si Si

O

O

O

O

O

Si O

O

OH

O

OH

O O HO O

O Si

Si Si

Si O

O HO

O

Si

Si Si

Si O

O Si

Si O

Si

Si Si

Si OH O Si O

O Si Si

O

O

O

O

Si OH O Si O

O OH

Si

Si

Si

O

O Si

Si

Si Si

O

O

O

+

NH3

NH2

+

NH3

+

NH3

+

O O O O

O

O

CH2 CH2

+

NH3

+

Si

O

CH2 CH2

+

NH3

NH3

O

O

CH2

CH2 CH2

+

O O HO

CH2

CH2

CH2

O

CH2

CH2

CH2

CH2

Si Si

Si CH2

Si

CH2

CH2

O HO O

+

Si

Si Si

Si Si

CH2O

CH2

NH2

+

NH3 CH2 CH 2

CH2

SiSi

Si Si Si

NH3

O

O

CH2

Si

Si

CH2 CH2 +

CH2

NH3 CH2

CH2

Si

Si

Si

+

NH3

CH2 CH2

+

CH2 Si

Si Si

Si

Si

NH2

O

O

NH2 CH2

CH2

O

Si Si

Si Si

O

CH2 CH CH2 2

NH2 +

CH2

CH2 Si Si Si

CH2 Si Si CH2

CH2

NH3

CH2

CH2

CH2

NH3

CH2

CH2

CH2

CH2

Si CH2

CH2

+

NH3

+

NH3

NH3

CH2 CH2

CH2

CH2

Si

Si

Si

CH2

CH2

O

Si

+

CH2

CH2

Si CH2

Si Si

Si Si CH2 CH2 CH2O CH2 CH2 +

CH2

CH2

CH2 CH2

Si

CH2

CH2 NH2

Si

Si

Si

Si

NH3

+

NH3 CH2 CH 2

CH2

SiSi

O

O

O

Si

CH2

CH2 +

CH2

Si

Si

Si

Si

CH2

CH2 NH3 CH2

CH2

Si Si Si

Si

NH3

CH2

Si

Si

Si

CH2O

CH2 CH2

O

CH2

Si

Si

Si

Si

Si

Si

Si

NH3 CH2 CH 2

Si SiSi

Si

Si

CH2

CH2 CH CH2 2

+

NH3

+

Si

Si

Si

CH2

CH2

CH2

Si Si

Si

Si

+

+

+

CH2 CH2

O

Si Si

O

NH3

NH2

CH2

CH2

Si Si

CH2 Si Si CH2

CH2

CH2

Si

Si

O CH2

+

NH3

CH2 CH2

O

Si Si

CH2 Si Si CH2

CH2

O O

O

CH2

CH2

CH2 Si Si Si

CH2 CH CH2 2 + NH3

O O O HO

CH2

+

NH3

O

O

+ CH NH3 2

CH2

CH2 +

NH3

CH2

CH2 Si Si Si

CH2

CH2 NH2

NH2 CH2

CH2

CH2

CH2 +

NH3 + NH3

CH2

CH2 +

NH3

NH2 ++

NH NH33

CH2

NH3

+

+

NH3

+

NH3

CH2

CH2 CH2

+

NH CH2 3 CH2

CH2

CH2 NH2

CH2

CH2

CH2 CH2

CH2

CH2

O

Si CH2

Si

Si

CH2O

CH2

CH2

Si Si

Si

Si

Si

NH3

+

NH3 CH2 CH 2

CH2 Si

Si Si Si

Si CH2 CH2

NH3

O

CH2

Si SiSi

Si

Si

Si

CH2

O

O

CH2

CH2

Si

Si

Si

NH3

dNi/Ni

+

NH3

CH2 Si

Si

O

O

NH2 CH2

CH2

Si Si

Si

Si CH2

CH2

O

Si Si

CH2 Si

CH2 CH CH2 2

O

O

+

CH2 CH2

CH2 Si Si Si

+

NH3

O HO

NH3

CH2

CH2 CH2

+

NH3

NH3

O

NH2 +

NH3 NH3

CH2 CH2

CH2

O

+

NH3

CH2

CH2 CH2

+

CH2

CH2

CH2

O

O

CH2

CH2

CH2

+

2

CH2

CH2

O

Si CH2

Si

NH2

O

CH2

Si Si

Si

Si Si

CH2O

CH2

O

O

O

+

Si

Si

Si

Si

NH3 CH2 CH 2

CH2

SiSi

Si

Si

CH2 CH2 +

O

CH2

Si

Si

Si

Si

NH3

O HO

+

NH3 CH2

CH2

Si Si Si

Si Si

O

CH2 CH CH2 2

+

NH3

O

NH2

CH2 CH2

O

Si Si

CH2 Si Si CH2

CH2

CH2

CH2

CH2

O HO

CH2

CH2

O

+

NH3

CH2

NH3

+

NH3

+

NH3

O

O

+

NH3

CH2

+

NH3

CH2 CH2

CH2

O

O

OH

O

O

O O O O

O O

OH

O O O

O

O HO

O

O HO

O O O

O O

O

O

O HO

O

O

O HO

O

O

O O

O

O O

O

On the other hand, the fact that mainly HOSiD absorption, but not its luminescence quantum yield and lifetime are significantly perturbed by the addition of AuNP indicates that no affinity between HOSiD and AuNP bring the particles in close proximity. The compact Au27 aggregates formed upon addition of HOSiD may be due to DLVO-type effects increasing the Van der Waals forces between Au27 particles, as a consequence of the increase in the ionic strength due to the presence of multivalent, negatively charged HOSiD and its co-ions. Such observations are in line with reported mechanisms on the aggregation of gold nanoparticles.44,47 A coarse representation of Au27 aggregates thus formed in the presence of HOSiD is schematized in Scheme 2B. Because of electrostatic repulsion forces between negative multivalent HOSiD and AuNP, there will be a local depletion of HOSiD close to AuNP surface (see Scheme 2C) at distances strongly depending on AuNP and SiD surface charge densities and solution ionic force.48 According 28 ACS Paragon Plus Environment

Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to our results, mean distances between Au27 aggregates and HOSiD are long enough to minimize the electromagnetic interaction between both particles; however, HOSiD still sense an enhanced electric field. CONCLUSIONS In this communication we have shown that silicon quantum dots can produce enhanced emission intensities by gold nanoparticles in a way similar to that of organic fluorophores. Furthermore, the MEF and enhanced absorption strongly depends on the nature of SiD surface groups, which ultimately govern the SiD-AuNP interactions. HOSiD and PASiD, both show similar absorption and emission spectra, as well as similar emission decays, characteristic of mildly oxidized Si semiconductor dots. Their emission spectrum, though not their absorbance, partially overlaps the plasmon band of Au27 and Au88. However, the different affinity of HOSiD and PASiD surface groups (silanols and propylamine, respectively) for citrate-capped gold surfaces not only controls the regime at which MEF takes place but also leads to the formation of Au27 aggregates of different morphology. It is discussed in the literature that the fluorescence brightness of molecular chromophores involving metal nanoparticles (MNP) strongly depends on the formation of particle aggregates.8 For a chromophore between two MNP, both non-radiative and radiative rate constants are enhanced at most distances between MNP. The radiative enhancement benefits from the coherent sum of molecular and induced dipoles which strengthen emission and absorption, while nonradiative processes originated in energy transfer between the chromophore and the MNP is less significant. This is indeed the situation observed for PASiD-Au27 ensembles.



On the other hand, PASiD-Au88 and HOSiD-Au88 ensembles can be thought in a regime similar to that of a single MNP and a chromophore for which an enhanced brightness is predicted when the molecule is at a particular distance from the metal surface, of ca. 10-20 nm for molecular chromophores. The different behavior of these ensembles may be envisaged on the basis of the electrostatic interactions between negatively charged Au88 surfaces and either NH3+ or Si-O- surface groups, respectively.17,18 However, the main distances involved in these processes which determine the occurrence of MEF may be smaller than the 10-20 nm expected for molecular chromophores. In fact, enhanced luminescence is reported in the literature for uncharged SiD in very close proximity to AuNPs encapsulated in a polymer matrix11 and for CdSe/ZnS quantum dots bound to gold nanorods through organic spacers13. The investigation herein presented showed strong evidence on the role played by the nature of SiD surface groups and surface charge on SiD MEF in colloidal aqueous suspension. However, a detailed understanding of the results still requires a full comprehension of plasmon-exciton interactions leading to enhanced luminescence. Overall these results are of importance in the design of new biomedical devices using SiD as luminescence sensors and therapeutic agents in technological applications.15 ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:.


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The PDF file contains the List of Reactants, Equipment, Attenuation and Total Transmittance Spectra, EDS of PASiD-Au27 ensembles, XPS survey, Luminescence Spectra, and Corrections for Inner Filter Effects. ACKNOWLEDGEMENTS J.J.R. thanks CONICET, Argentina, for a graduate studentship. MCG, HBR, and JHH are research members of CONICET. The work was performed with funds of the grants PICT 2014-2746 and 2014-2153 from ANPCyT, Argentina. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

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Silicon Quantum Dots Metal Enhanced Photoluminescence by Gold

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