RESEARCH ARTICLE
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In elligen Magne ic Mic o obo s wi h Fluo escen In e nal
Memo y o Moni o ing In agas ic Acidi y
N. Sen hilna han, Caga ay M. O al, Adam No obilsky, and Ma in Pume a*
This s udy in es iga es he dynamic fluc ua ions o pH caused by gas ic acid
sec e ion, a p ocess o bo h biological and clinical significance, wi h
mic o obo s. Abno mal pa e ns o acidi y o en indica e gas oin es inal
diseases, unde lying he impo ance o p ecise in agas ic pH moni o ing.
T adi ional me hods using fluo escen p obes ace challenges due o hei
ain solid-s a e fluo escence, limi ed a ge specifici y, and accu acy. To
o e come hese obs acles, pH- esponsi e fluo escen o ganic mic opa icles
deco a ed wi h magne i e (Fe3O4) nanopa icles a e enginee ed. These
mic o obo s exhibi a unique fluo escence swi ching capabili y a a c i ical pH,
enabling he moni o ing o gas ic acidi y. The magne ic pa o hese
mic o obo s ensu es magne ic maneu e abili y o enable a ge ed na iga ion.
The mic o obo s’ fluo escence swi ching mechanism is elucida ed h ough
comp ehensi e spec oscopy, mic oscopy, and X- ay diff ac ion analyses,
e ealing molecula -le el s uc u al ans o ma ions upon in e ac ion wi h
gas ic acid and an acids. These ans o ma ions, specifically p o ona ion and
dep o ona ion o he mic o obo s’ fluo escen componen s, p omp a dis inc
fluo escence esponse co ela ing wi h pH shi s. In i o and ex i o
expe imen s, simula ing s omach condi ions, confi m he mic o obo s’
efficacy in pH- esponsi e imaging. The esul s showcase he p omising
diagnos ic po en ial o mic o obo s o gas oin es inal ac diseases,
ma king a significan ad ancemen in imaging-based medical diagnos ics a
a ge ed loca ions.
N. Sen hilna han, C. M. O al, M. Pume a
Fu u e Ene gy and Inno a ion Labo a o y
Cen al Eu opean Ins i u e o Technology
B no Uni e si y o Technology
Pu kyˇ
no a 123, B no 61200, Czech Republic
E-mail: [email p o ec ed].cz
A.No obilsky
Depa men o Pha macologyandToxicology
Ve e ina yResea chIns i u e
Hudco a296/70,B no62100,CzechRepublic
The ORCID iden ifica ion numbe (s) o he au ho (s) o his a icle
can be ound unde h ps://doi.o g/10.1002/ad m.202401463
© 2024 The Au ho s. Ad anced Func ional Ma e ials published by
Wiley-VCH GmbH. This is an open access a icle unde he e ms o he
C ea i e Commons A ibu ion License, which pe mi s use, dis ibu ion
and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly
ci ed.
DOI: 10.1002/ad m.202401463
1. In oduc ion
Gas ic acid, p ima ily composed o hy-
d ochlo ic acid, pepsin, and lipase, plays
a i al ole in ood diges ion by b eak-
ing down p o eins in o se e al polypep-
ides and amino acids.[1]pH o gas ic
acid in he ange o 1.5–3 is c i ical o
a heal hy diges i e sys em and p o ides
significan p o ec ion om mic oo gan-
isms and pa hogenic in ec ions in oduced
h ough inges ed ood.[2,3]Imbalances in
gas ic acid sec e ion lead o a ious gas-
oin es inal diso de s, including gas oe-
sophageal eflux disease (GERD) and pep-
ic ulce s. P o on pump inhibi o s (PPIs)
and an acids a e he mos commonly used
medica ions o acid eflux ea men o
con ol acid sec e ion and hei acidi y,
espec i ely.[4]To assess he efficacy o
hese medica ions owa d gas ic diso de s,
moni o ing in agas ic acidi y is highly
c i ical.[5,6]Al hough adi ional echniques,
such as esophageal pH es ing and he Hei-
delbe g capsule me hod, offe insigh ul in-
o ma ion abou he pH changes o he
gas ic acid, mos o hese me hods a e
insensi i e and in asi e, lacking eal- ime
M. Pume a
Ad anced Nano obo s & Mul iscale Robo ics Labo a o y
Facul y o Elec ical Enginee ing and Compu e Science
VSB – Technical Uni e si y o Os a a
17. lis opadu 2172/15, Os a a 70800, Czech Republic
M. Pume a
Depa men o Medical Resea ch
China Medical Uni e si y Hospi al
China Medical Uni e si y
No. 91 Hsueh-Shih Road, Taichung 40402, Taiwan
M. Pume a
Depa men o Chemical and Biomolecula Enginee ing
Yonsei Uni e si y
50 Yonsei- o, Seodaemun-gu, Seoul 03722, Sou h Ko ea
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moni o ing capabili ies and causing discom o o he
pa ien .[7,8]
pH- esponsi e fluo escen molecula ma e ials, polyme s,
quan um do s, and nanopa icles wi h high sensi i i y ha e been
widely used in bio- ela ed applica ions, including con olled d ug
deli e y, he apeu ic applica ions, and pH sensing wi hin cells,
o ganelles, issues, and li ing animals.[9–12]Fluo escen molecu-
la ma e ials s and ou among he o he ma e ials due o hei
s uc u al and fluo escence ailo abili y, and hey a e u he
appealing due o hei excellen he mal-, chemical-, and pho-
os abili y. Howe e , i is wo h no ing ha mos fluo escen
molecula ma e ials suffe om weak solid-s a e fluo escence, a
phenomenon known as “agg ega ion-caused quenching” (ACQ),
and lack o a ge selec i i y, limi ing hei easibili y in sensing
applica ions.[13]Ins ead o ocusing solely on designing molec-
ula ma e ials wi h high a ge selec i i y o bio-sensing appli-
ca ions, an al e na i e and effec i e s a egy has been adop ed in
his s udy o de ise molecula ma e ials-based mic o obo s wi h
emo ely con olled na iga ion ea u es in esponse o ex e nal
s imuli. This app oach no only simplifies he design p ocess bu
also p o es o be highly effec i e o a ge ed sensing o physio-
logical changes wi hin he bio-sys em.[14]
The u iliza ion o mic o/nano obo s in he biomedical field is
cons an ly ad ancing[15–18]as hey possess significan po en ial
in he medical sec o o eplace con en ional e he ed obo ic sys-
ems. The abili y o mic o obo s o na iga e in ha d- o-ope a e a -
eas o he body highligh s hei p ominence as un e he ed small-
scale su gical ins umen s owa d non-in asi e diagnosis and
he apy.[19–21]Among he mic o obo s ha can exhibi locomo-
ion unde diffe en s imuli, such as ligh i adia ion,[22–24]mag-
ne ic field,[25–27]elec ic field,[28,29]and acous ic wa es,[30–32]mag-
ne ically d i en mic o obo s a e p ominen candida es o bio-
ela ed applica ions because o hei p ecisely con ollable uel-
ee na iga ion ea u es, e en in high pene a ion dep hs, wi h-
ou damaging he issue.[33–35]Ou esea ch g oup has success-
ully de eloped such mic o obo s o a ious biomedical and en-
i onmen al emedia ion applica ions, including biofilm e adi-
ca ion, p os a e cance he apy, and he emo al o pollu an s
om wa e (mic o/nanoplas ics, ni oa oma ic explosi es, and
o ganic oxic pollu an s).[36–41]Al hough magne ic mic o obo s
a e p omising candida es o hese applica ions, he equi emen
o acking can be conside ed a significan challenge. In his
ega d, fluo escence emission is a highly sensi i e and powe -
ul ool ha can play a pi o al ole in bo h acking and sens-
ing applica ions wi hin biological sys ems. I s significance lies in
hei non-in asi e measu emen possibili y wi hou dis u bing
he physiological unc ions o li ing sys ems. P e iously, ou e-
sea ch g oup has c a ed fluo escen mic o obo s o o ganic dye
deg ada ion and me al ion de ec ion applica ions.[42,43]Fluo es-
cen mic o obo s ha e widesp ead use in di e se biomedical ap-
plica ions, such as imaging-guided d ug deli e y and ela ed he -
apeu ic in e en ions.[44–47]Fo ins ance, a ew s udies in ol ing
zinc- and magnesium-based uel- ee mic o obo s ha e been e-
po ed o gas ic acid neu aliza ion he apy in he gas oin es i-
nal (GI) ac .[48–50]Recen ly, me al-based mic omo o s coa ed
wi h comme cially exis ing NIR-II fluo escen dyes ha e been
epo ed as p o on- esponsi e fluo escen p obes specifically de-
signed o GIimaging.
[51]He e,wedesignedanewse o mul i-
fluo escen molecula ma e ials-based mic o obo s o moni o
gas ic acid pH changes. To he bes o ou knowledge, o
moni o ing he acidi y o in agas ic acid, fluo escen molecula
ma e ial-based mic o obo s ha e no been epo ed p e iously.
Ou choice o using diaminodicyanoquinodime hanes (DADQs)
is no ewo hy due o hei pH-dependen fluo escence swi ch-
ing p ope ies[52,53]and s ong solid-s a e fluo escence emission,
popula ly known as “agg ega ion-induced emission (AIE)”.[54]
Fu he mo e, DADQ de i a i es ha e been epo ed as bioma k-
e s o he bac e ial spo es and s oma al cells o plan s, highligh -
ing hei sui abili y o bio- ela ed applica ions.[55,56]
In he p esen s udy, 7,7-bis(2-(2-aminoe hyl)py idino)−8,8-
dicyanoquinodime hane (BAP) molecules wi h blue (BAP-B),
g een (BAP-G), and o ange (BAP-O) fluo escence a e syn hesized
o he ab ica ion o Fe3O4-deco a ed mul i-fluo escen mag-
ne ic mic o obo s. In his mic o obo design, fluo escen BAP
molecules a e he p ima y building block o he mic o obo s, and
Fe3O4nanopa icles a e inco po a ed o magne ize he mic o-
obo s o acili a e mo ion, na iga ion, and collec abili y. In addi-
ion, spec oscopic and mic oscopic analyses p o ide insigh in o
he molecula changes associa ed wi h he mic o obo s’ fluo es-
cence swi ching. As illus a ed in Scheme 1, fluo escence imag-
ing s udies success ully demons a e he cyclic pH a ia ions o
gas ic acid by moni o ing he fluo escence changes o he fluo-
escen mic o obo s a diffe en pH condi ions. Such signaling
can pa e he way o he u iliza ion o fluo escen mic o obo s o
diagnose GI ac in ec ions. The fluo escen mic o obo s ecog-
nize he pH fluc ua ions and can also “ ecall” he acidic pH hey
passed h ough —e en i hey e u n o he neu al pH solu ion—
by adia ing dis inc colo s o a specific du a ion (Scheme 1c).
Since memo y is one o he key elemen s o in elligence, we in-
oduce mobile in elligen mic o obo s in his s udy.
2. Resul s and Discussion
2.1. Fab ica ion and Cha ac e iza ion
Polymo phs o BAP molecules wi h diffe en fluo escence
we e syn hesized by adop ing diffe en syn he ic condi-
ions. BAP-B was syn hesized by he di ec addi ion o 2-(2-
aminoe hyl)py idine in o he ace oni ile solu ion o e a-
cyanoquinodime hane (TCNQ) a 72 °C o 3h(Figu e 1a).[52]
Con e sely, BAP-G was syn hesized in e ahyd o u an (THF)
om he same eac an s a 66 °C o 3h.1HNMRspec ao
bo h BAP-B and BAP-G se ed as an e idence o hei s uc u al
in eg i y and pu i y (Figu e S1, Suppo ing In o ma ion). The
molecula s uc u e and uni cell molecula assembly o BAP-B
and BAP-G c ys als we e de e mined using single-c ys al X- ay
diff ac ion analysis (Figu e 1b–e). Single-c ys al X- ay diff ac-
ion analysis u he e eals ha BAP-B and BAP-G molecules
exhibi cha ac e is ic a e age dihed al angles o 46.11°and
42.35°, espec i ely, be ween he benzenoid co e ing and di-
aminome hylene o BAP moie y (Tables S1, S2, Suppo ing
In o ma ion). BAP-B c ys allizes as a P212121space g oup in
i s ace oni ile solu ion,[52]whe eas BAP-G c ys allizes as a
P21/c space g oup in he DMSO–wa e sol en mix u e. The
H-bonding in e linked sup amolecula assembly o BAP-B and
BAP-G molecules in he c ys als is depic ed in Figu es S2 and
S3 (Suppo ing In o ma ion). On he o he hand, BAP-O was
p epa ed by ea ing pu e BAP-G c ys als wi h ifluo oace ic
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Scheme 1. Schema ic ep esen a ion o he mul i-fluo escen magne ic mic o obo s o moni o ing cyclic pH a ia ions in gas ic acid. a) Mul i-
fluo escen BAP (7,7-bis(2-(2-aminoe hyl)py idino)−8,8-dicyanoquinodime hane)-based mic o obo s. b) Magne ic collec abili y and ac ua ion o he
mic o obo s unde ex e nal magne ic fields. c) Fluo escence swi ching o BAP mic o obo s a diffe en pH. d) Fluo escence imaging o BAP-O mic o-
obo s a diffe en pH in so issue sandwich model and ex i o fluo escence imaging o mice be o e (con ol) and a e injec ing BAP-O mic o obo s.
acid o an ex ended du a ion,[57]as ou lined in he Ma e ials and
Me hods sec ion (s uc u al ea u es a e shown in Figu e S4a,
Suppo ing In o ma ion). The 1H NMR spec um o c ude BAP-
O e eals he e en ion o cha ac e is ic peaks obse ed in BAP-G
wi h a downfield shi , along wi h he eme gence o new p o on
signals o igina ing om a achmen s o he dicyanome hylene
ca bon (𝛿=5.30 ppm) and py idine g oups (𝛿=4.15 ppm)
as illus a ed in Figu e S4b (Suppo ing In o ma ion). The 19F
NMR spec um p o es he p esence o ifluo oace ic acid moi-
e y in he c ude BAP-O (Figu e S4c, Suppo ing In o ma ion).
The esul s demons a e ha BAP-O is he p o ona ed o m o
BAP-G, and pos -pu ifica ion, i s peak posi ions a e consis en
wi h hose o pu e BAP-G (Figu e S5, Suppo ing In o ma ion).
Elec onic abso p ion and emission spec a o all BAP
molecules a e shown in Figu e 1 ,g, espec i ely. In he solid
s a e, BAP-B, BAP-G, and BAP-O molecules exhibi abso p-
ion maxima a 360, 385, and 385 nm wi h a shoulde peak a
522 nm, espec i ely, while in ace oni ile solu ion, all molecules
abso b he maximum a a ound 385 nm. The obse ed signifi-
can blueshi in he solid abso p ion spec um o BAP-B can be
a ibu ed o he push–pull molecula en i onmen . This shi e-
sul s om he an ipa allel o ien a ion o neighbo ing molecula
dipoles, c ea ing a localized elec ic field ha ele a es he exci ed
ene gy le els, consequen ly inc easing he S0-S1ene gy gap.[52,54]
The emission maxima o BAP-B, BAP-G, and BAP-O molecules
in he solu ion s a e a e obse ed a 477, 480, and 583 nm, e-
spec i ely. In con as , in he solid s a e, hei maximum emis-
sions occu a 447, 493, and 583 nm, espec i ely. In addi ion o
he blueshi obse ed in he solid-s a e emission spec a, he e
is a ema kable enhancemen in he in ensi y o solid-s a e emis-
sions o ha o solu ion-s a e emission a an equi alen op ical
densi y. The fluo escence enhancemen s a e likely due o he
hind ance o in amolecula and in e molecula non- adia i e ex-
ci ed s a e ene gy decay. The fluo opho es wi h s ong solid-s a e
fluo escence emission a e p omising candida es o bio- ela ed
applica ions, and his enhancemen phenomenon is well-known
as “agg ega ion-induced emission” (AIE).[58]The fluo escence
emission o BAP-G in he sol en mix u e o wa e and THF
exhibi s significan enhancemen when he wa e ac ion is in-
c eased om 0% o 95%, as depic ed in Figu e S6 (Suppo ing In-
o ma ion), highligh ing i s agg ega ion-induced emissi e p op-
e y.
BAP-B, BAP-G, and BAP-O mic opa icles ha ing deco a-
ion o Fe3O4nanopa icles we e ab ica ed using a simple
ep ecipi a ion me hod, as shown in Figu e S7a (Suppo ing In-
o ma ion). Fe3O4nanopa icles a e employed o magne ize he
mic opa icles and he eby acili a e hei mo ion in he p esence
o a magne ic field, and hus called as mic o obo s. Acco ding o
field emission scanning elec on mic oscopy (FESEM) images
(Figu e 2a), BAP-B, BAP-G, and BAP-O mic opa icles ha e
simila mo phologies a e inco po a ing Fe3O4nanopa icles,
as depic ed in Figu es S7b–m (Suppo ing In o ma ion), and he
size o he mic o obo s is ound o be in he ange o 500 nm o
6μm. Ene gy-dispe si e X- ay spec oscopy (EDX) mapping con-
fi ms he success ul a achmen o Fe3O4nanopa icles on he
su ace o he ca bon-based mic o obo s du ing he ab ica ion
p ocess. This a achmen is e iden om he p esence o i on and
oxygen elemen s, as illus a ed in Figu e 2b. Space g oup a ia-
ions be ween BAP-B and BAP-G molecules a e likely o induce
dis inc molecula sel -assembly p ocesses and pa icle g ow h
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Figu e 1. Syn hesis and cha ac e iza ion o 7,7-bis(2-(2-aminoe hyl)py idino)−8,8-dicyanoquinodime hane (BAP). a) Schema ic ep esen a ion o he
syn hesis o BAP-B and BAP-G. b,d) Molecula s uc u e and c,e) uni cell molecula assembly o b,c) BAP-B, and d,e) BAP-G de e mined using single-
c ys al X- ay diff ac ion analysis, whe e C: g ey, N: blue, and H: ligh g ay. ) Abso p ion and g) emission spec a o BAP-B (Ex: 350 nm), BAP-G (Ex:
350 nm), and BAP-O (Ex: 520 nm) in solid- (con inuous lines) and solu ion-s a e (do ed lines). Solu ion-s a e emission spec a o BAP molecules we e
mul iplied by a ac o o 100 o imp o e hei isibili y.
mechanisms, leading o he o ma ion o pa icles wi h di e se
mo phologies and su ace oughness. These cha ac e is ics play
a c ucial ole in a ying deg ees o a achmen o Fe3O4nanopa -
icles on he su aces o bo h BAP-B and BAP-G mic opa icles,
as shown in Figu es 2a and S7 (Suppo ing In o ma ion).
No ably, he p o ona ed s a e o BAP-O mic o obo s exhibi s
enhanced a ac ion owa ds Fe3O4nanopa icles, as depic ed in
Figu e 2a, hanks o he ionic in e ac ions. Fluo escence mic o-
scopic images in Figu e 2c demons a e he mul i-fluo escen
emissi e ea u es o mic o obo s, and i is impo an o highligh
ha he obse ed fluo escence colo s o he mic o obo s a e
consis en wi h hei emission spec a. The impac o he ab-
ica ion p ocess on he molecula s uc u es o BAP molecules
in he s uc u e o mic o obo s was analyzed using Fou ie -
ans o m in a ed (FTIR) and powde X- ay diff ac ion (PXRD)
s udies (Figu es 2d–g and S8 (Suppo ing In o ma ion)). PXRD
pa e ns o as-p epa ed BAP-B, BAP-G, and BAP-O, as well as
he co esponding mic o obo s, indica e ha all hese samples
exhibi a c ys alline na u e. The simula ed X- ay diff ac ion
pa e ns o BAP-B and BAP-G a e closely ma ched wi h hose
o hei as-p epa ed samples and mic o obo s. Addi ionally,
in he FTIR spec a, cha ac e is ic peaks, and peak posi ions
o as-p epa ed BAP-B and BAP-G emain unchanged o he
mic o obo s, sugges ing he absence o s uc u al modifica ions
du ing he ab ica ion p ocess. Howe e , in he p epa a ion o
BAP-O, new peaks eme ged a 1677, 1632, and 1198 cm−1,along
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Figu e 2. Cha ac e iza ion o BAP mic o obo s. a) FESEM images o he BAP-B, BAP-G, and BAP-O mic o obo s (Scale ba s: 1 μm). b) EDAX analy-
sis o an indi idual BAP-G mic o obo deco a ed wi h Fe3O4nanopa icles, showing he p esence o ca bon, i on, and oxygen elemen s (Scale ba s:
2μm). c) Fluo escen mic oscopy images o BAP-B, BAP-G, and BAP-O mic o obo s (Scale ba s: 5 μm). d, ) PXRD and e,g) FTIR spec a o as-p epa ed
mic oc ys als and mic o obo s o d,e) BAP-B and ,g) BAP-O. A simula ed XRD pa e n o BAB-B is also shown o e e ence.
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Figu e 3. Collec abili y and ac ua ion o mic o obo s unde ex e nal magne ic fields. a) Pho og aphs o aqueous dispe sions o BAP-G mic o obo s unde
UV ligh (i) be o e and (ii) a e he magne ic collec ion. b) Pic o ial ep esen a ion o he cus om-buil magne ic se up used o mo ion expe imen s. c)
A e age speed and d) displacemen o BAP-G mic o obo s a diffe en equencies unde a magne ic field o 5 mT (Scale ba : 20 μm). e) Magne ically
con olled na iga ion o a BAP-G mic o obo on he p edesigned “BAP ack” (Scale ba : 20 μm).
wi h cha ac e is ic peaks o as-p epa ed BAP-G. The peaks o
BAP-O a 1677 and 1198 cm−1a e a ibu ed o he p o ona ion
o py idine and he p esence o C─F bond unc ionali ies ( om
he ifluo oace ic acid), espec i ely. The eme gence o new
peaks p o es once again ha BAP-O is he p o ona ed o m o
BAP-G. No ably, he cha ac e is ic peaks o Fe3O4nanopa icles
a e no obse ed in he spec a o any BAP mic o obo s due o
hei limi ed p esence compa ed o BAP molecules.
2.2. Magne ic Ac ua ion
The magne ic p ope ies o he as-p epa ed BAP-G and he co -
esponding mic o obo s we e assessed h ough ib a ing sample
magne ome e (VSM) analysis, and he esul ing magne ic hys-
e esis loops a e epo ed in Figu e S9 (Suppo ing In o ma ion).
The esul s e eal ha ba e BAP-G mic opa icles ha e a negli-
gible magne ic esponse, while he magne ic p ope ies signifi-
can ly change o BAP-G mic o obo s due o he p esence o Su-
pe pa amagne ic Fe3O4nanopa icles. Addi ionally, he magne ic
collec abili y o he mic o obo s was es ed using a neodymium–
i on–bo on (NdFeB) magne close o BAP mic o obo s, as shown
in Figu e 3a, and he images we e cap u ed unde he illumina-
ion o UV ligh . In Figu e 3a(i), he dispe sion o mic o obo s in
an aqueous medium is depic ed, while Figu e 3a(ii) illus a es he
mic o obo s being a ac ed owa d he magne . Mic o obo s ha
exhibi mo ion wi h localiza ion capabili y wi hin animal bod-
ies unde he influence o a magne ic field highligh significan
p omise o imaging applica ions in li ing o ganisms.[59]The e-
o e, magne ically induced locomo ion o he BAP-G and BAP-O
mic o obo s (Mo ies S1 and S2, espec i ely (Suppo ing In o -
ma ion)) and hei con olled na iga ion (Mo ie S3, Suppo ing
In o ma ion) we e ho oughly analyzed using a cus omized mag-
ne ic se up equipped wi h an op ical mic oscope and a sophis i-
ca ed con olling uni . In his se up, h ee o hogonal coil pai s
we e employed o gene a e a ans e sal o a ing magne ic field
o induce he mo ion o mic o obo s (Figu e 3b). To de e mine
he s ep-ou equency o he mic o obo s, a e age speed alues
we e calcula ed in he ange o 10–80 Hz while keeping he mag-
ne ic field as 5 mT along he x-axis (Figu e 3c). The s ep-ou e-
quency o BAP-G mic o obo s is iden ified as 30 Hz a 5 mT, as
depic ed in Figu e 3c,d. A 30 Hz, he mic o obo s achie ed hei
maximum speed o 8 μms
−1, and a e age speed alues dec eased
beyond his equency. The dec ease in speed alues is a ibu ed
o he loss o mic o obo s’ synch oniza ion wi h he highe mag-
ne ic equency, which is a esul o he inc eased iscous o que
exe ed by he su ounding liquid.[60,61]Al hough he e a e a y-
ing le els o Fe3O4nanopa icle a achmen obse ed on BAP-B
and BAP-G mic o obo s as shown in Figu es 2a and S7 (Suppo -
ing In o ma ion), hei speed emains ela i ely same a a ound
8μms
−1. In con as , he highe concen a ion o Fe3O4nanopa -
icles adhe ed o he su ace o BAP-O mic o obo s esul ed in
an enhanced ac ua ion speed o app oxima ely 10 μms
−1,su -
passing ha o bo h BAP-B and BAP-G mic o obo s. Apa om
speed, con olled na iga ion is also a c i ical ea u e o mic o-
obo s o hei bio- ela ed applica ions and i is app aised by ob-
se ing hei mo emen upon changing he di ec ion o he mag-
ne ic field using a con olle . Figu e 3e and Mo ie S3 (Suppo ing
In o ma ion) illus a e ha a single mic o obo can be na iga ed
along a p edefined “BAP ack” by only adjus ing he di ec ion
o he ex e nal magne ic field (5 mT, 20 Hz). The as esponse
and he emo ely con ollable na iga ion o magne ic BAP mic o-
obo s make hem a p omising candida e o a ious bio-medical
applica ions, such as a ge ed diagnosis, moni o ing, and nano-
su ge y.
2.3. Fluo escence P ope ies
The pH-dependen op ical cha ac e is ics o BAP-B, BAP-G, and
BAP-O mic o obo s we e comp ehensi ely in es iga ed unde
a ious condi ions. Elec onic abso p ion and fluo escence
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Figu e 4. pH-dependen fluo escence swi ching o mic o obo s and he memo y effec . a) Va ia ion o he fluo escence in ensi y o BAP-B, BAP-G, and
BAP-O mic o obo s a diffe en pH. b) Abso p ion and c) emission spec a (Ex: 350 nm) o BAP-B mic o obo s in wa e and gas ic fluid (GF), ollowed
by he addi ion o an acid. Pho og aphs o d) BAP-B, e) BAP-G, and ) BAP-O mic o obo s a diffe en pH condi ions: ini ially in hei o iginal s a e in
wa e (pH 7) and hen a e he addi ion o gas ic fluid (GF, pH 2), and finally ollowing he subsequen addi ion o an acid (AA, pH 7). A plausible
mechanism o BAP-B’s p o ona ion and dep o ona ion a e he addi ion o gas ic acid and an acid is p o ided below he image (d). All he pho og aphs
a e aken unde UV ligh (365 nm) illumina ion.
emission spec a o he aqueous dispe sion o all BAP mic o-
obo s we e eco ded a diffe en pH, as depic ed in Figu e
S10 (Suppo ing In o ma ion), and he esul s a e summa-
ized in Figu e 4a. The esul s indica e ha he fluo escence
emission in ensi y o all BAP mic o obo s in he aqueous dispe -
sions/solu ions dec eases as he acidi y o he solu ion inc eases.
In o he wo ds, he fluo escen BAP mic o obo s and hei fluo-
escence emission a e s able in he pH ange o 4–7, and he flu-
o escence emission in ensi y expe iences a significan all when
he pH d ops below 3 due o hei significan dissolu ion. These
ini ial findings offe p omising p ospec s o moni o ing pH
changes o gas ic acid (pH 1−3) by employing he fluo escen
BAP mic o obo s. In he p ocess o moni o ing pH changes o
gas ic acid, he ollowing se ies o expe imen s we e conduc ed.
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Ini ially, BAP-B mic o obo s we e subjec ed o gas ic acid and
subsequen ly ea ed wi h an an acid (neu alize ), as shown in
Figu e 4d– , o unde s and hei fluo escence swi ching p op-
e ies. Blue fluo escen (BAP-B) mic o obo s lose hei fluo es-
cence when ea ed wi h gas ic acid due o hei dissolu ion; he
los fluo escence is immedia ely egained a e adding an an acid
due o he e o ma ion o BAP mic opa icles (Figu e 4d). Du ing
he dissolu ion p ocess in gas ic acid, BAP pa icles in he mi-
c o obo s a e dissol ed, and hen deco a ed Fe3O4nanopa icles
a e de ached om he s uc u e o he mic o obo s. Mos o he
nanopa icles a e no ea ached o he su ace o BAP mic opa i-
cles du ing he e o ma ion p ocess a e he addi ion o an acid.
In e es ingly, he egained fluo escence emission appea s g een,
bu o e se e al hou s, i g adually e u ns o i s o iginal blue flu-
o escence. This ansi ion is one o he exci ing ea u es obse ed
in he p ocess o moni o ing pH changes in gas ic acid, which
was named as “fluo escence memo y.” Simila expe imen s
we e also ca ied ou wi h BAP-G and BAP-O mic o obo s, and
he esul s a e summa ized in Figu e 4e, . In he case o BAP-G
mic o obo s, los fluo escence ollowing exposu e o gas ic acid
is apidly es o ed o a g eenish-yellow fluo escence upon he
addi ion o an an acid. Subsequen ly, i is ans o med back o
i s o iginal g een fluo escence a e se e al hou s (Figu e 4e).
The unde lying easons o hese ansi ions we e elucida ed
using FTIR and PXRD analyses and discussed la e . On he
con a y, he “fluo escence memo y” was no obse ed in BAP-O
mic o obo s, and i s o iginal fluo escence was egained
Immedia ely a e he addi ion o an an acid (Figu e 4 ). This
diffe ence in he beha io o BAP-O mic o obo s om BAP-B
and BAP-G mic o obo s is likely a ibu ed o he s uc u al mod-
ifica ions unde gone by BAP-O du ing i s exposu e o ifluo-
oace ic acid. I is impo an o no e ha BAP-B and BAP-G we e
syn hesized om he same eac an s unde diffe en condi ions,
whe eas BAP-O is he p o ona ed o m o BAP-G as depic ed in
Figu e S4 (Suppo ing In o ma ion).
The fluo escence swi ching o he mic o obo s in esponse o
pH changes is also alida ed h ough spec oscopic analysis. The
elec onic abso p ion and fluo escence emission spec a o ba e
mic o obo s in wa e , gas ic acid, and ollowing he addi ion o
an an acid a e depic ed in Figu es 4b,c. Figu e 4b illus a es ha
BAP-B mic o obo s consis en ly display an abso p ion maxima
a 355 nm in all he cases. E en hough BAP-B mic o obo s ha e
simila op ical densi ies (highligh ing he same concen a ion)
in all he cases, hey exhibi diffe en fluo escence emission wi h
in ensi y a ia ions, as shown in Figu e 4c, due o he molecula
s uc u al changes ha occu in he BAP-B moie y upon he addi-
ion o gas ic acid and an acid. Ini ially, he aqueous dispe sion
o BAP-B mic o obo s exhibi s an emission maximum a 444 nm,
consis en wi h ha o he mic oc ys alline solid o m. Howe e ,
upon he addi ion o gas ic acid, he fluo escence peak almos
disappea s due o he p o ona ion o BAP-B. The subsequen ad-
di ion o an acid, specifically sodium bica bona e, es o es hem
o hei dep o ona ed s a e, he eby b inging back he fluo es-
cence peak a 485 nm wi h a no able ed-shi o 40 nm. The ed-
shi ed fluo escence peak g adually e u ns o i s o iginal posi-
ion a 444 nm o e ime. In simple e ms, a ain g een fluo es-
cence peak ans o ms in o a i id blue fluo escence peak wi hin
a span o 6 hou s, and his ansi ion was p e iously e e ed o
as “fluo escence memo y” ansi ion. A simila expe imen was
epea ed wi h BAP-G and BAP-O mic o obo s, yielding simila
ou comes o BAP-G bu no o BAP-O. The abso p ion and flu-
o escence emission maxima o he aqueous dispe sion o BAP-G
mic o obo s a e obse ed a 355 and 492 nm, espec i ely, con-
sis en wi h ha o hei mic oc ys alline. The fluo escence peak
o BAP-G disappea s upon he addi ion o gas ic acid, and i is
es o ed a e he in oduc ion o he an acid. No ably, he fluo es-
cence memo y ansi ion is no significan ly obse ed in he case
o BAP-G and no obse ed a all o BAP-O. As shown in Figu e
S11 (Suppo ing In o ma ion), fluo escence emission peaks o
BAP-O be o e and a e he addi ion o gas ic acid/an acid is no
swi ched significan ly. The lack o a no iceable change indica es
he absence o a fluo escence memo y ansi ion o BAP-O mi-
c o obo s.
To gain deepe insigh s in o molecula -le el changes unde -
lying he fluo escence memo y ansi ion o BAP mic o obo s,
FTIR and PXRD analyses we e ca ied ou (Figu es S12–S14,
Suppo ing In o ma ion). In he FTIR analysis, ema kably, no
changes we e isible in he ib a ional equencies o key unc-
ional moie ies o all BAP mic o obo s, e en a e he addi ion o
gas ic acid/an acid, as shown in Figu es S12–S14a (Suppo ing
In o ma ion). The absence o molecula s uc u al changes sug-
ges s ha he fluo escence memo y ansi ion may occu due o
some o he ac o s. In o de o e eal i , PXRD analysis was ca -
ied ou (Figu e S12–S14b, Suppo ing In o ma ion). The PXRD
analysis un eils ha he e e sible c ys al-amo phous ansi ion
ha occu s in BAP mic o obo s leads o hei fluo escence mem-
o y ansi ions. The diff ac ion pa e ns clea ly e eal he ini-
ial c ys alline na u e o as-p epa ed BAP-B, BAP-G, and BAP-
O, as well as he co esponding mic o obo s. As illus a ed in
he diff ac ion pa e ns, excep o BAP-O mic o obo s, he o he
wo mic o obo s lose hei c ys allini y upon he addi ion o gas-
ic acid and an acid, which explains he absence o a fluo es-
cence memo y ansi ion o BAP-O mic o obo s. In iguingly,
hey g adually e-es ablish hei c ys allini y wi hin a 6-hou pe-
iod.
2.4. Fluo escence Imaging
The p ocess o p ac ical implemen a ion o moni o ing pH fluc-
ua ions in gas ic acid using BAP fluo escen mic o obo s was
ini ia ed wi h a comp ehensi e in i o imaging analysis. In hese
expe imen s, equal quan i ies o ba e BAP-B, BAP-G, and BAP-
O mic o obo s we e indi idually placed in well pla es con ain-
ing wa e , gas ic acid, and a mix u e o gas ic acid and an acid,
as illus a ed in Figu e 5a, and fluo escence imaging was con-
duc ed using app op ia e exci a ion and emission wa eleng hs.
As shown in Figu e 5a, BAP mic o obo s display s ong fluo es-
cence in he wells con aining wa e o a mix u e o gas ic acid
and an acid. Howe e , hey exhibi no fluo escence emission in
he gas ic acid wells. These esul s sugges ha all BAP mic o-
obo s possess he capabili y o measu e in agas ic acidi y, mak-
ing hem sui able o a di e se ange o in i o in es iga ions.
Fu he mo e, a so issue model (Figu e 5b) was also u ilized o
in es iga e he imaging pe o mance o mic o obo s when hey
a e p esen below a issue. Fo hese expe imen s, aqueous sus-
pensions o BAP-B, BAP-G, and BAP-O mic o obo s we e sepa-
a ely injec ed in o he issue sandwich model filled wi h 0.5 mL
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Figu e 5. Fluo escence imaging. a) Schema ic desc ip ion and fluo escence images o BAP-B, BAP-G, and BAP-O mic o obo s in wa e , in gas ic fluid
(GF), and in a mix u e o GF and an acid (AA). b) Schema ic desc ip ion o a so issue model u ilized du ing imaging expe imen s and fluo escence
esponse o BAP-O mic o obo s in wa e , in GF, and ollowed by he addi ion o AA in o he issue model. c,d) Random dis ibu ion and magne ic
collec abili y o BAP-O mic o obo s in he issue model. e) Ex i o fluo escence imaging o mouse be o e (le ) and a e ( igh ) injec ion o BAP-O
mic o obo s (injec ion loca ion is shown wi h an a ow). Ex: 410 nm and Em: 535 nm o BAP-B and BAP-G mic o obo s. Ex: 410 nm and Em: 600 nm
o BAP-O mic o obo s.
o wa e , and hen imaged unde he sui able exci a ion wa e-
leng hs. As an impo an obse a ion, he fluo escence emi ed
by BAP-B and BAP-G mic o obo s was no de ec able in he is-
sue model (Figu e S15, Suppo ing In o ma ion). This lack o de-
ec ion is a ibu ed o he in e e ence and o e lap wi h he au -
ofluo escence o he issues u ilized o he expe imen s. How-
e e , in con as o BAP-B and BAP-G mic o obo s, BAP-O mi-
c o obo s we e success ully able o emi de ec able fluo escence
in he issue model (Figu e 5b). This diffe ence could be a esul
o he emission ange o BAP-O mic o obo s a he han he au -
ofluo escence o he issue, enabling hei dis inc iden ifica ion
and moni o ing capabili ies in a complex en i onmen .[62]La e ,
an aqueous dispe sion o BAP-O mic o obo s was injec ed in o
he issue model filled wi h 0.5 mL o gas ic acid (0.01 M), and
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