nanoma e ials
A icle
NIR-Emi ing Alloyed CdTeSe QDs and O ganic Dye
Assemblies: A Non oxic, S able, and E icien
FRET Sys em
Do is E. Ramí ez-He e a 1, Eus olia Rod íguez-Velázquez 2,3, Manuel Ala o e-Meda 3,4,
F ancisco Pa aguay-Delgado 5, An onio Ti ado-Guíza 1, Pablo Taboada 6
and Geo gina Pina-Luis 1,*
1
Cen o de G aduados e In es igación, Ins i u o Tecnológico de Tijuana, A.P. 1166, 22500 Tijuana, BC, Mexico;
[email p o ec ed] (D.E.R.-H.); [email p o ec ed] (A.T.-G.)
2Facul ad de Odon ología, Uni e sidad Au ónoma de Baja Cali o nia, Calzada Uni e sidad 14418,
Pa que Indus ial In e nacional, 22390 Tijuana, BC, Mexico; eus olia. [email p o ec ed]
3Ins i u o de O opedia y Banco de Tejidos Musculoesquelé icos, Uni e sidad de San iago de Compos ela,
Campus Su S/N, E-15782 San iago de Compos ela, Spain
4CONACyT—Ins i u o Tecnológico de Tijuana, Cen o de G aduados e In es igación en Química, Bl d.
Albe o Limón Padilla S/N, 22510 Tijuana, BC, Mexico; [email p o ec ed]
5Cen o de In es igación en Ma e iales A anzados S. C., Depa amen o de Física de Ma e iales, A . Miguel
de Ce an es 120, Complejo Indus ial Chihuahua, CP 31109 Chihuahua, Chih., Mexico;
ancisco.pa aguay@cima .edu.mx
6G upo de Física de Coloides y Políme os, Depa amen o de Física de Ma e ia Condensada,
Facul ad de Física, Uni e sidad de San iago de Compos ela, Campus Su S/N,
E-15782 San iago de Compos ela, Spain; [email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +52-664-6233-772; Fax: +52-664-6234-341
Recei ed: 28 Feb ua y 2018; Accep ed: 30 Ma ch 2018; Published: 11 Ap il 2018
Abs ac :
In he p esen wo k, we syn hesize Nea In a ed (NIR)-emi ing alloyed me cap op opionic
acid (MPA)-capped CdTeSe quan um do s (QDs) in a single-s ep one-hou p ocess, wi hou he
use o an ine a mosphe e o any py opho ic ligands. The quan um do s a e wa e soluble,
non- oxic, and highly pho os able and ha e high quan um yields (QYs) up o 84%. The alloyed
MPA-capped CdTeSe QDs exhibi a ed-shi ed emission, whose colo can be uned be ween isible
and NIR egions (608–750 nm) by con olling he Te:Se mola a io in he p ecu so mix u es and/o
changing he ime eac ion. The MPA-capped QDs we e cha ac e ized by UV- isible abso p ion
spec oscopy, luo escence spec oscopy, ansmission elec on mic oscopy (TEM), ene gy dispe si e
X- ay spec oscopy (EDS), and ze a po en ial measu emen s. Pho os abili y s udies we e pe o med
by i adia ing he QDs wi h a high-powe xenon lamp. The e na y MPA-CdTeSe QDs showed g ea e
pho os abili y han he co esponding bina y MPA-CdTe QDs. We epo he Fö s e esonance ene gy
ans e (FRET) om he MPA-capped CdTeSe QDs as ene gy dono s and Cyanine5 NHS-es e (Cy5)
dye as an ene gy accep o wi h e iciency (E) up o 95%. The dis ance be ween he QDs and dye ( ),
he Fö s e dis ance (R0), and he binding cons an (K) a e epo ed. Addi ionally, cy ocompa ibili y
and cell in e naliza ion expe imen s conduc ed on human cance cells (HeLa) cells e ealed ha
alloyed MPA-capped CdTeSe QDs a e mo e cy ocompa ible han MPA-capped CdTe QDs and a e
capable o o de ing homogeneously all o e he cy oplasm, which allows hei use as po en ial sa e,
g een dono s o biological FRET applica ions.
Keywo ds:
ene gy ans e ; alloyed quan um do s; quan um do s-dye assemblies; cy ocompa ibili y
Nanoma e ials 2018,8, 231; doi:10.3390/nano8040231 www.mdpi.com/jou nal/nanoma e ials
Nanoma e ials 2018,8, 231 2 o 14
1. In oduc ion
Quan um do s (QDs) a e conside ed an excellen al e na i e o o ganic luo opho es due o hei
unique op ical p ope ies, such as b oad exci a ion spec a, high pho os abili y, size- unable emission,
na ow and symme ic emission spec a, and high quan um yields (QYs) [
1
]. These cha ac e is ics
demons a e hem as a ac i e ma e ials o biomedical and echnological applica ions, such as clinical
diagnosis and labeling [2–4], as well as o sola ene gy con e sion in pho o ol aic de ices [5,6].
Rega ding hei en a i e use in he biomedical ield, QDs a e well-known o espond in he ed
o nea -in a ed (NIR) window (600–900 nm), which is highly ad an ageous o
in i o
and
in i o
imaging and de ec ion [
7
]. NIR ligh easily pene a es animal issue due o he minimum abso bance o
NIR pho ons [
8
]. Te na y alloyed QDs ha e ecen ly eme ged as a new class o NIR emi e s a ac ing
conside able a en ion, due o he capabili y o uning hei op ical emission wi hou changing he
pa icle size. Examples o e na y NIR QDs a e CuInS
2
, AgInSe
2
, and CdTeSe [
9
]. Se e al g oups ha e
in es iga ed he syn hesis o NIR-emi ing CdTeSe QDs [
10
–
14
]. Mos o hese s udies a e based on
co e-shell QD syn hesis, since his ype o QD shows g ea e pho os abili y and QYs, bu hei syn hesis
is complica ed and ime-consuming. Because o his, a sys ema ic s udy o ob ain only co e-doped
QDs wi h high QYs and pho os abili y is equi ed.
Syn he ic me hods in ol ing o ganic sol en s such as ioc ylphosphine (TOP) and ioc ylphosphine
oxide (TOPO) [
15
] ha e p oduced QDs soluble in non-pola sol en s exclusi ely. This condi ion limi s
hei applica ion in he biomedical a ena. Su ace modi ica ion ep esen s an al e na i e syn he ic
s a egy o ob ain wa e -soluble and less oxic QDs [
16
,
17
]; howe e , i is known ha ligand su ace
modi ica ion ails o main ain he high QYs o QDs. An impo an s a egy is he di ec syn hesis
o wa e -soluble QDs, in a single s ep, elimina ing he supe icial exchange o ligands ha leads
o signi ican dec eases o he QY. This s a egy has been used in he hyd o he mal syn hesis o
wa e -soluble bina y [
18
,
19
] and e na y [
20
] QDs wi h good esul s, using me cap o acids as s abilize s.
In pa icula , Ma e al. [
18
,
19
] p epa ed high-quali y wa e -soluble CdTe QDs and in es iga ed he
in luence o di e en me cap o acids on he g ow h a e, size dis ibu ion, luo escence, and s abili y
o he QDs. Liu e al. [
20
] p esen ed e na y me cap op opionic acid (MPA)-capped CuInS
2
QDs wi h
QYs o 3.3% and s udied he in luence o a ious expe imen al a iables, including he p ecu so
concen a ions, eac ion ime, eac ion empe a u e, pH alue, and capping ligand used on he
luminescen p ope ies o he ob ained QDs. These e na y QDs we e used o label li e cance cells.
Ano he impo an aspec o conside is he possible oxici y o his ype o QD. Some QDs
ha e been ound o be cy o oxic only a e oxida i e and/o pho oly ic deg ada ion o hei co e
coa ings, demons a ing ha co e co e g ea ly imp o es he biocompa ibili y o QDs wi h no obse ed
cy o oxici y e en a e y high concen a ion and long- ime exposu e in cells [
21
]. Se e al s a egies
ha e been de eloped o op imize QD s abili y and biocompa ibili y. Te na y alloyed QDs ep esen
a p omising al e na i e due o hei excellen op oelec onic p ope ies such as: highe QY, unable
emission wa eleng h con olling Te:Se s oichiome y wi hou signi ican change in he pa icle size,
wide abso p ion ange, highe chemical and s uc u al s abili ies due o dec eased in e di usion,
and ha dened la ice s uc u e [
22
]. These ad an ages may con ibu e o a dec ease in QD cy o oxici y.
The s abili y and cy o oxici y o alloyed QDs ha e been less s udied han bina y QDs. Mos cy o oxici y
app oaches and cell in e naliza ion s udies ha e been pe o med on co e-shell QDs [
23
–
26
], while
he po en ial cy o oxici y o e na y alloyed QDs emains o be in es iga ed ho oughly [
27
,
28
].
The e o e, we begin his wo k by ca ying ou a sys ema ic s udy o ob ain wa e -soluble shell-less
alloyed MPA-coa ed QDs, in a single s ep, wi h high QYs and pho os abili y. An impo an issue is o
de e mine whe he he hi d elemen in e na y QDs in luences hei oxicological p ope ies, as well
as he in luence o su ace coa ing, hyd odynamic diame e , and he pa icle su ace cha ge.
On he o he hand, Fö s e esonance ene gy ans e (FRET) mechanism has been widely used in
di e en applica ions and cons i u es he base o a new gene a ion o luminescen senso s. QDs ha e
shown g ea po en ial as luo opho es in FRET-based sensing applica ions, and hey ha e been widely
adop ed as ei he ene gy dono s o accep o s [
29
,
30
]. Al hough he e a e nume ous applica ions o
Nanoma e ials 2018,8, 231 3 o 14
QD-based FRET p obes, as a as we know, no s udies ha e been epo ed demons a ing he use o
e na y MPA-capped CdTeSe QDs in FRET sys ems.
In his wo k, we epo a sys ema ic s udy on he syn hesis o high QY NIR-emi ing MPA-capped
CdTeSe QDs wi hou he use o an ine a mosphe e and py opho ic ligands in a single s ep and in less
han an hou . Pho os abili y s udies we e pe o med o e alua e he change o luo escence p ope ies.
We used a model based on MPA-capped CdTeSe QDs as ene gy dono s and Cyanine5 NHS-es e
(Cy5) dye as an ene gy accep o in FRET assays. A pH 7, bo h species a e linked by elec os a ic
in e ac ions. The e ec o QD size on he FRET p ocess be ween nanopa icles and dye was s udied.
Finally, cy o oxici y and cell in e naliza ion o bina y CdTe and e na y CdTeSe QD o mula ions we e
es ed upon incuba ion wi h human cance cells (HeLa).
2. Ma e ials and Me hods
2.1. Ma e ials and Cha ac e iza ion
All ma e ials used in he cu en wo k we e eagen s o analy ical g ade. Cadmium chlo ide
hemi(pen ahyd a e) (CdCl
2
+ 2.5H
2
O, 81%), po assium ellu i e (K
2
TeO
3
, 90+%), 3-me cap op opionic
acid (MPA, 99+%), sodium bo ohyd ide (NaBH
4
, 99.99+%), sodium hyd oxide (97%), Cyanine5 NHS-es e
(Cy5), and luo escein we e pu chased om Sigma-Ald ich Chemicals Co. (Toluca, Edo. de Mexico, Mexico).
O he eagen s (analy ical g ade) and sol en s (spec oscopic g ade) we e pu chased om Sigma-Ald ich
Chemicals Co. (Toluca, Edo. de Mexico, Mexico).
The exci a ion and emission spec a we e eco ded on a Ho iba NanoLog luo escence
spec opho ome e (Kyo o, Japan) using a Xe lamp as he exci a ion sou ce. QY was de e mined
using luo escein wi h a QY o 79% in NaOH 0.1 M as a e e ence. The abso p ion spec a we e
measu ed using a Va ian Ca y-100 UV- isible spec opho ome e (San a Cla a, CA, USA). A Jeol
JEM2200FS ansmission elec on mic oscope (TEM) (Akishima, Japan) was used o examine he
appea ance and size o nanopa icles. The mic oscope has sphe ical abe a ion co ec ion in a scanning
ansmission elec on mic oscopy (STEM mode, Akishima, Japan) wo king a an accele a ing ol age
o 200 KeV. The elemen al composi ion was de e mined by ene gy dispe si e spec oscopy (EDS)
Ox o d (Abingdon, UK), in which he quali a i e elemen al analysis was made in he STEM mode.
Ze a po en ial was ob ained on a Ho iba Scien i ic SZ-100 (Kyo o, Japan) nanopa icle analyze . All pH
measu emen s we e made wi h a The mo Scien i ic pH me e (Wal ham, MA, USA).
2.2. Syn hesis o MPA-Capped CdTeSe QDs
Wa e -soluble and highly luo escen MPA-capped CdTe and CdTeSe nanoc ys als we e
syn hesized acco ding o he p e iously desc ibed me hod wi h some modi ica ions [
31
].
B ie ly, MPA (0.4 mmol) and CdCl
2
+ 2.5H
2
O (0.4 mmol) we e dissol ed in 100 mL deionized wa e
in a h ee-necked lask. Unde magne ic s i ing, he pH o he mix u e was adjus ed o 10 by using
he d op-wise addi ion o NaOH solu ion (1 M). A e 5 min o igo ous s i ing, 100 mL o a solu ion
p epa ed om K
2
TeO
3
and elemen al Se o di e en mola a ios o Te:Se (1:0, 0.75:0.25, and 0.50:0.50)
and NaBH
4
(4.2 mmol) we e added in o he mix u e. A e ano he 5 min o s i ing, he lask was
a ached o a condense and e luxed a 100
◦
C. Aliquo s we e aken om he solu ion a di e en
e luxing imes (30, 45, and 60 min) o ob ain QDs o di e en sizes. The as-p epa ed MPA-capped
QD solu ions we e concen a ed and pu i ied by e hanol p ecipi a ion, collec ed ia cen i uga ion a
3300 pm, and e-dispe sed in wa e . Concen a ion o QDs was es ima ed spec opho ome ically [
32
].
The QYs o he syn he ized MPA-capped QDs we e de e mined h ough Equa ion (1):
φx=φSTG adx
G adST n2
x
n2
ST !(1)
whe e he subsc ip s ST and xdeno e s anda d and es , espec i ely,
φ
is he luo escence quan um
yield, G ad he g adien om he plo o in eg a ed luo escence in ensi y e sus abso bance, and n he
Nanoma e ials 2018,8, 231 4 o 14
e ac i e index o he sol en . The s anda d sample was luo escein wi h a QY o 79% in NaOH
solu ion (0.1 M).
2.3. MPA-Capped CdTeSe QDs and Dye Conjuga es
Dono MPA-capped CdTeSe QDs we e chosen acco ding o he spec al o e lap o hei emission
spec a wi h he abso p ion spec a o he accep o Cy5 dye. The pH o he solu ion was main ained a
7 using 2-(N-mo pholino)e hanesul onic acid (MES)-bu e (10 mM). A his pH alue, he dono and
accep o ha e nega i e and posi i e su ace cha ge, espec i ely.
2.4. Ti a ion o MPA-CdTeSe QDs wi h Cy5 Dye
Cyanine5 NHS-es e (Cy5) dye s ock solu ions we e p epa ed in wa e (100
µ
M). 3 mL o
QD solu ion (0.5
µ
M) we e i a ed by successi e addi ion o dye s ock solu ion unde s i ing and
hei esul ing emission spec a we e eco ded.
2.5. CCK-8 Assay
The cy ocompa ibili y o MPA-capped CdTe and CdTeSe QDs was de e mined h ough he
CCK-8 assay. The CCK-8 is a sensi i e colo ime ic es ha allows a apid quan i ica ion o
me abolically ac i e cells in a cul u e, which consis s o he educ ion o a WST-8 e azolium sal in o a
wa e -soluble o mazan p oduc (ligh b own in colo ). The amoun o he colo ed o mazan p oduc is
p opo ional o he numbe o iable cells [
33
]. B ie ly, HeLa cells wi h an op ical con luence o 80–90%
we e seeded in o 96-well pla es (100
µ
L, 1.5
×
10
4
cells/well) and g own o 11 h a s anda d cul u e
condi ions (5% CO
2
a 37
◦
C) in Dulbecco’s Modi ied Eagle Medium (DMEM) supplemen ed wi h
10% e al cal se um (FCS), 2 mM L-glu amine, 1% penicillin/s ep omycin, 1 mM sodium py u a e,
and 0.1 mM MEM non-essen ial amino acids (NEAA). A e wa ds, he MPA-capped QDs, dissol ed
in 10 mM phospha e bu e ed saline (PBS), we e pipe ed in o he cell-con aining wells (100
µ
L, a
a ying concen a ions) and incuba ed o 8 h. The medium con aining he MPA-capped QDs (200
µ
L)
was exchanged wi h esh medium (100
µ
L), and he cells we e u he incuba ed o comple e 24 h.
Then, he cul u e medium was disca ded, and he cells we e added o a esh cul u e medium (100
µ
L)
con aining he CCK-8 eagen (10
µ
L), ollowed by gen le shaking o 1 min and incuba ion o up o 3
h a s anda d cul u e condi ions. The op ical densi y (OD) o he o mazan was measu ed a e wa d a
450 nm using an ELISA mic opla e eade (BIO-RAD model 680, He cules, CA, USA). The me abolic
ac i i y o he cells a e exposu e o he MPA-capped QDs, ep esen ed as he pe cen age o cell
iabili y, was calcula ed by no malizing he o mazan OD eading om he cells exposed o he
MPA-capped QDs ega ding con ol, non-exposed cells (100% iabili y). The esul s a e he a e age o
six independen expe imen s.
2.6. Con ocal Mic oscopy
The cellula in e naliza ion o he MPA-capped CdTeSe QDs was eco ded by using
a LEICA TCS-SP5 con ocal mic oscope (LEICA Mic osys ems Heidelbe g GmbH, We zla ,
Hesse, Ge many) equipped wi h a blue diode (
λ
= 405 nm) and a pulsed whi e line lase
(WLL, o
λ
= 488, 561 and 633 nm). The cells we e seeded on poly-L-lysine coa ed glass co e slips
(12
×
12 mm) placed inside 6-well pla es (3 mL, 5
×
10
4
cells/well) and g own o 24 h a s anda d
cul u e condi ions. A e wa ds, he MPA-capped CdTeSe QDs a he desi ed concen a ion we e
in e nalized (200
µ
L, 10
−9
M), and he incuba ion p o ocol was conduc ed as desc ibed o he
cy ocompa ibili y expe imen s. A 24 h o incuba ion, he MPA-CdTeSe QDs-con aining cells we e
washed and s ained o isualiza ion wi h DAPI (cell nucleus) and Bodipy Phalloidin (cy oplasm).
B ie ly, 350 mL o pa a o maldehyde (4% w/ in 1
×
PBS) we e added o each co e slip, incuba ed
o 10 min, and washed wi h 1
×
PBS. Then, 350 mL o a T i on X-100 solu ion (0.1% w/ in
1
×
PBS) we e added, incuba ed o 10 min, and washed wi h 1
×
PBS. Then, 200
µ
L o Bodipy
Phalloidin 650/665 (5% / in 1
×
PBS) we e added, incuba ed o 20 min in da kness, and washed
Nanoma e ials 2018,8, 231 5 o 14
wi h 1
×
PBS. Then, he a ached cells we e exposed o one d op o P oLong Gold an i ade loaded
wi h DAPI and cu ed o 24 h a
−
20
◦
C in da kness. Exci a ion lines o 405 (emissions a 425–475 nm)
and 633 nm (emissions a 650–725 nm) we e employed o he isualiza ion o cellula nucleus (s ained
wi h 4
0
,6-diamidino-2-phenylindole (DAPI), om In i ogen, Ca lsbad, CA, USA) and cy oplasm
(s ained wi h Bodipy Phalloidin om In i ogen, Ca lsbad, NM, USA), espec i ely. All expe imen s
we e ca ied ou in duplica e.
3. Resul s and Discussion
3.1. Syn hesis o Nea -In a ed MPA-Capped CdTe and CdTeSe QDs
Two impo an challenges in he syn hesis o QDs o hei applica ion in biological sys ems ha e
been o c ea e QDs ha (1) a e biocompa ible and (2) emi in he nea in a ed. Howe e , many o he
syn he ic me hods epo ed using o ganic sol en s, which in ol es a second s ep o he modi ica ion
o he QD su ace wi h hiola ed ligands o make i wa e soluble. Al hough hiol-based ligands
ha e p o en s able su ace ligands o long- e m wa e solubili y, hey ail o conse e he high QYs
o QDs [
34
,
35
]. The e is always a signi ican educ ion in he luminescence e iciency ollowing he
ansi ion om o ganic sol en s in o wa e (QY is usually below 20%) [
36
]. Fo his eason, he di ec
aqueous syn hesis o MPA-capped QDs using a single-s ep and ambien a mosphe e me hodology was
pe o med. The syn hesis condi ions we e modi ied wi h he objec i e o op imize he QY o he QDs.
Then, QDs o CdTeSe we e ob ained a di e en mola a ios o Te:Se (1:0, 0.75:0.25, and 0.50:0.50) and
di e en eac ion imes (15, 30, 60, and 90 min). Figu e S1 shows images o he as-p epa ed QDs unde
ambien ligh and UV ligh . A clea ed-shi in he emission wa eleng h is seen o longe eac ion
imes, indica ing ha la ge QDs ha e o med.
Cha ac e iza ion by UV-VIS and Fluo escence Spec oscopy
Abso p ion and luo escence spec a o he MPA-capped QDs e-dispe sed in wa e a pH 8 we e
ob ained. As can be app ecia ed in Figu e 1, when he eac ion ime inc eases and he Te p opo ion
is lowe , he emission wa eleng hs shi o he nea -in a ed zone. Unlike bina y QDs, he g ow h
kine ics o alloyed CdTeSe QDs a e dependen upon h ee chemical eac an s (Cd, Te, and Se) and
hei ela i e concen a ions in he eac ion [
12
]. I has been p o en ha elemen al Te is conside ably
mo e eac i e han Se owa d Cd unde apid nuclea ion and ha equilib ium be ween Te and Se a e
eached a highe eac ion imes [
10
]. The change in he Te and Se composi ion du ing QD g ow h was
e lec ed on he QDs emission wa eleng hs.
Figu e 1.
E olu ion o (
a
) abso p ion and (
b
) luo escence spec a o me cap op opionic acid
(MPA)-capped CdTeSe quan um do s (QDs) in wa e a pH 8 ob ained a di e en mola a ios o Te:Se
(1:0, 0.75:0.25, 0.50:0.50) and di e en ime eac ions.
Nanoma e ials 2018,8, 231 6 o 14
The mechanism o he QDs’ o ma ion can be desc ibed as ollows; du ing he i s s ep MPA
coo dina es wi h Cd
2+
ions o o m he Cd-MPA complex and o p e en he deposi ion o non-soluble
Cd compounds. The addi ion o NaBH
4
esul s in he educ ion o TeO
32−
o Te
2−
and he educ ion
o elemen al Se o Se
2−
. The p ecu so o Te (Te
2−
) apidly eac s ( wice as as as Se
2−
) esul ing
CdTe- ich nuclei QDs ini ially. As he eac ion p og esses, CdSe is o med on he co e, and an alloyed
s uc u e is p oduced. MPA ligands igh ly bind o he QDs’ su aces.
Simila o he change in he emission wa eleng h, QYs become la ge as he eac ion ime inc eases
om 15 o 60 min and he Te:Se mola a io dec eases (Table S1). The highes QY, 84%, was ob ained
wi h a mola ela ion o 0.75:0.25 o Se:Te and a eac ion ime o 60 min. These a e ema kable
esul s since ew in es iga ions ha e ob ained compa able QY alues o wa e -soluble QDs [
37
].
The MPA-capped QDs’ diame e s we e es ima ed using he abso p ion peaks and he empi ical
equa ion p e iously epo ed by Peng [
32
] a di e en eac ion imes (see Table S1). STEM images o
he p epa ed MPA-capped CdTe and CdTeSe QDs (Figu e 2) indica e uni o mly dispe sed, sphe ical
QDs. The esul s ob ained om he analysis o he size dis ibu ion his og ams o he QDs show ha
he a e age c ys al size was 3.4
±
0.23, 3.8
±
0.25, and 4.1
±
0.2 nm, con i ming he esul s ob ained by
he Peng eg ession.
Nanopa icle composi ion was u he examined by EDS analysis. The p esence o Cd, Te, and Se
was con i med by he p esence o cha ac e is ic peaks o hese elemen s (Figu e S2). The emaining
signals co espond o la ice elemen s o he sample suppo .
Figu e 2.
Scanning ansmission elec on mic oscopy (STEM) images ob ained by Z con as o he
MPA-capped QDs a di e en mola a ios o Te:Se (1:0, 0.75:0.25, 0.50:0.50) and ime eac ion o 60 min.
Abo e: he co esponding his og am o size dis ibu ion.
Mo eo e , he ob ained QDs dispe sed in he phospha e bu e (pH 8) showed high op ical
s abili y when i adia ed unde xenon ion lase a 365 nm UV ligh o 60 min. Figu e 3shows he
dependence o he ela i e luo escence in ensi y o he MPA-capped CdTe and MPA-capped CdTeSe
QDs wi h he UV i adia ion ime. P e ious s udies ha e shown ha unsa u a ed Te and Se a oms on
QDs’ su aces may be oxidized unde UV ligh exposu e, which may lead o possible quenching o
QDs’ luo escence [
38
]. The as-p epa ed QDs (bina y and e na y) showed no dec ease in luo escence
in ensi y. In ac , a pho o enhancemen o 15% was obse ed o he MPA-capped CdTe QDs in he i s
10 minu es, while he luo escence o he alloyed MPA-capped CdTeSe emained p ac ically cons an
h oughou he i adia ion ime in e al. The di e ences in luminescence can he e o e be a ibu ed
o di e en su ace p ope ies [
6
]. The alloyed MPA-capped CdTeSe QDs wi h g adien dis ibu ion
o componen s consis ing o Te- ich inne co es and Se- ich ou e shells con ain ewe ap si es o
quenching de ec s on he su ace.
Nanoma e ials 2018,8, 231 7 o 14
The pho o b igh ening o he MPA-capped QDs a e UV i adia ion has been associa ed wi h he
pho o-adso p ion o wa e molecules on o nanoc ys als’ su aces p o iding es o ing unc ions on he
QDs’ su aces, elimina ing pho oluminescence (PL)-quenching de ec s o ap si es [
39
]. These esul s
con i m ha he e a e ewe ap si es o quenching de ec s on he su aces o e na y QDs. Based on
his beha io , he MPA-capped CdTeSe QDs we e selec ed o u he FRET s udies.
Figu e 3.
Pho os abili y o he MPA-capped CdTe and he MPA-capped CdTeSe QDs in wa e a pH 8
unde UV ligh i adia ion.
3.2. Fö s e Resonance Ene gy T ans e FRET S udies
The po en ial o QDs as luo escen ene gy dono s in FRET assays was explo ed by designing
ou sys ems o MPA-capped CdTeSe QDs (QDs626, QDs629, QDs636, and QDs663) as ene gy dono s
and Cy5 dye as ene gy accep o s. Acco ding o he Fö s e esonan ene gy ans e heo y, a condi ion
ha a ec s he e iciency o ene gy ans e is he o e lap be ween he dono emission spec um and
he accep o abso p ion spec um. The abso p ion spec um o he Cy5 dye and emission spec a o
he ou MPA-capped CdTeSe QDs (spec a a e no malized) a e shown in Figu e 4. As can be seen,
he emission spec a o he MPA-capped CdTeSe QDs and he abso p ion spec um o he Cy5 showed
a la ge spec al o e lap.
Figu e 4.
Spec al o e lap o he dono MPA-capped CdTeSe QDs emissions and he accep o Cy5
dye abso p ion. The solid lines ep esen he emission spec a o he dono s (
λem
= 626, 629, 636,
and 671 nm) and he do ed line ep esen s he abso p ion spec um o he Cy5 dye (λem = 639 nm).
Nanoma e ials 2018,8, 231 8 o 14
3.3. Selec ion o he Op imal Condi ions o he FRET P ocess
The op imal wa eleng h o exci e he FRET pai is he one o which only he dono is exci ed,
and he accep o exci a ion is minimal. As can be obse ed in he abso p ion spec um o he dye in
he Figu e 5a, his wa eleng h is 450 nm. In Figu e 5b, a h ee dimensional (3D) spec um p oduced
o he MPA-capped CdTeSe and he dye solu ion wi h a mola a io o 1:20, espec i ely, is shown.
The 3D spec a in Figu e 5b show ha he op imum exci a ion wa eleng h o he QDs is a 450 nm,
while he op imum exci a ion wa eleng h o he dye is 630 nm.
Figu e 5.
(
a
) Abso p ion spec um o he Cy5 dye and (
b
) he MPA-capped CdTeSe-Cy5 conjuga e
3D spec a.
3.4. Ze a Po en ial
Wi h he objec i e o co obo a e he su ace cha ge on he QDs and he elec os a ic in e ac ions
be ween he MPA-capped CdTeSe QDs and he Cy5 dye, Z po en ial was de e mined by a ying he
pH o he QDs (Figu e 6a) and adding successi e amoun s o he Cy5 dye (Figu e 6b).
Figu e 6.
(
a
) Po en ial Z o he MPA-CdTeSe QDs a di e en pH alues and (
b
) he i a ion o he
MPA-CdTeSe QDs wi h he Cy5 a pH 8.
The MPA-capped QDs ha e nega i e cha ge be ween pH 4–11 (Figu e 6a), due o ca boxyla e
g oups on hei su aces. An isoelec ic poin is localized a pH 4 coinciding wi h he pKa o he
ca boxylic acid. The successi e addi ions o he dye o he MPA-QDs, lead o a dec ease in he
QDs’ nega i e su ace cha ge (
−
60 m ) upon addi ion o inc easing amoun s o he Cy5 dye, indica ing
complex o ma ion (Figu e 6b). This beha io con i ms he elec os a ic in e ac ions be ween he
posi i ely-cha ged dye and he nega i ely-cha ged nanopa icles, dec easing he o al cha ge o he
Nanoma e ials 2018,8, 231 9 o 14
MPA-QDs. The MPA-capped CdTeSe QDs bond wi h he Cy5 dye ia elec os a ic in e ac ions be ween
hei ca boxyla e g oups and he iminium g oup o Cy5 dye. In he phospha e bu e (pH 8), s able
conjuga es a e o med.
3.5. MPA-Capped CdTeSe QDs’ Ti a ions o Di e en Sizes wi h Cy5 Dye
The in luence o he QDs’ sizes on he FRET p ocess was s udied by successi e i a ions o
di e en sizes o QDs (QDs626, QDs629, QDs636, and QDs671) wi h he Cy5 dye. A dec ease in he
QDs’ emissions and he enhancemen o he Cy5 dye emission was only seen o QDs629 and QDs636,
indica ing a local in e ac ion be ween he QDs and he Cy5 dye, as well as a FRET p ocess o hese
conjuga es. Fo he conjuga es be ween QDs626 and QDs663, = he ypical beha io o a FRET p ocess
was no shown. Figu e 7shows he emission spec a o he successi e i a ion o he MPA-capped
CdTeSe QDs (629 and 636) wi h he Cy5 dye (λex = 450 nm).
Figu e 7.
Ti a ion emission spec a o he MPA-capped CdTeSe QDs wi h he Cy5 dye. (
a
) QDs629
and (b) QDs636.
Figu e S3 illus a es he PL signals o he wo se s o 629- and 636-nm-emi ing QDs, as a unc ion
o he Cy5/QD mola a io. The plo s ag ee well wi h he expe imen al obse a ions o he p og essi e
