NL25 fold TPTK against unknown here?

[WantToLearn]

Hi 2+2, some more advice needed, thank you very much in advance!

<u>Question:</u> can I / must I fold this river against an unkown?

PokerStars No-Limit Hold'em, $0.25 BB (8 handed) Poker Stars Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

BB ($23.40)
UTG ($10.45)
Hero ($29.25)
MP1 ($31)
MP2 ($25)
CO ($15.25)
Button ($10.30)
SB ($14.75)

Preflop: Hero is UTG+1 with Q[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/club.gif[/img]. MP2 posts a blind of $0.25.
<font color="#666666">1 fold</font>, <font color="#CC3333">Hero raises to $1</font>, <font color="#666666">1 fold</font>, MP2 (poster) calls $0.75, <font color="#666666">4 folds</font>.

Flop: ($2.35) 7[img]/images/graemlins/spade.gif[/img], 5[img]/images/graemlins/heart.gif[/img], 6[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $2</font>, MP2 calls $2.

Turn: ($6.35) Q[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">MP2 bets $3</font>, Hero calls $3.

<font color="blue"> I decide to get passive here as he might very well think I am giving up a c-bet with air. </font>

River: ($12.35) 9[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">MP2 bets $7.25</font>, Hero ?